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Treatment of domestic wastewater using conventional and baffled septic tanks a
a
Fayza Aly Nasr & Basem Mikhaeil a
Water Pollution Research Department, National Research Centre, Cairo, Egypt Accepted author version posted online: 17 Jan 2013.Published online: 26 Feb 2013.
To cite this article: Fayza Aly Nasr & Basem Mikhaeil (2013) Treatment of domestic wastewater using conventional and baffled septic tanks, Environmental Technology, 34:16, 2337-2343, DOI: 10.1080/09593330.2013.767285 To link to this article: http://dx.doi.org/10.1080/09593330.2013.767285
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Environmental Technology, 2013 Vol. 34, No. 16, 2337–2343, http://dx.doi.org/10.1080/09593330.2013.767285
Treatment of domestic wastewater using conventional and baffled septic tanks Fayza Aly Nasr and Basem Mikhaeil∗ Water Pollution Research Department, National Research Centre, Cairo, Egypt
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(Received 5 January 2012; final version received 13 January 2013 ) The main theme of the study was a comparative study of domestic wastewater treatment using conventional and baffled septic tanks. The septic tanks were fed continuously with domestic wastewater at three different hydraulic retention times (HRTs). The HRTs chosen were 24, 48 and 72 h with corresponding organic loads of 0.321, 0.436 and 0.885 kg chemical oxygen demand (COD) per m3 per day, respectively. The performance of the septic tanks at the three HRTs gave satisfactory results. For the conventional septic tank, COD removal was 53.4%, 56% and 65.3%, at an HRT of 24, 48 and 72 h, respectively, with residual COD of 412, 380 and 334 mg/l, respectively. At HRTs of 72, 48 and 24 h, the following percentages removals were realized for: biochemical oxygen demand (BOD), 68.4%, 57, 53.5%; total suspended solid (TSS), 65.3%, 58.3, 55%; phosphorus, 29.3%, 26.9, 25.6%; total Kjeldahl nitrogen 26.8%, 20.8, 17.7%, respectively. On the contrary, ammonia concentrations increased by 7.1%, 5.2 and 4.2% under the same conditions. Consequently, the results showed that the removal of fecal coliform at all HRTs was less than one log. The two baffled septic tanks exhibited superior results at HRTs of 72, 48 and 24 h. Comparing the treated domestic wastewater quality produced by the two types of septic tanks in terms of physico-chemical and biological characteristics, better results were obtained using the two baffles type. Keywords: domestic; wastewater; treatment; conventional; baffled; septic; de-sludging.
1. Introduction Its considerable advantages mean that the simple septic tank system is the most commonly known primary treatment method for onsite wastewater treatment. Septic tanks remove most settleable solids and function as an anaerobic bioreactor that promotes partial digestion of organic matter. The main cause of their failure is the unsuitability of the soil and the site characteristics [1]. The system is inexpensive, and simple to operate and maintain, although sludge may cause an odour problem if left untreated for a long time [2]. Conventional onsite wastewater treatment systems are not effective in removing nitrate and phosphorus compounds and in reducing pathogenic organisms [3,4]. Septic tanks may be used alone or in combination with other processes to treat raw wastewater before it is discharged to a subsurface infiltration system. The tank provides primary treatment by creating quiescent conditions inside a covered, watertight rectangular, oval or cylindrical vessel, which is typically buried. In addition to primary treatment, the septic tank can reduce the sludge and scum volumes by as much as 40%. It also conditions the wastewater by hydrolysing organic molecules for subsequent treatment in the soil or by other unit processes. The outlet connections (e.g. a sanitary ‘tee’ fitting) retain the sludge and scum layers in the tank and draw effluent only from the clarified zone between the sludge and scum layers.
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
Septic tanks are used in nearly all onsite systems regardless of the daily wastewater flow rate or strength. The tanks provide suspended solids removal, solids storage and digestion [5]. Three zones are present in a septic tank: a scum layer which forms a crust on the surface of the tank liquor; the wastewater from which solids deposit; and a bottom sludge layer of deposited material. The organic matter in the tank may undergo anaerobic digestion. The degree of digestion depends on the tank size, frequency of cleaning and temperature. The capacity of the tank required is governed by the number of people it serves and desludging interval. Although a fraction of particulate solids are removed by flotation and sedimentation, nearly all entering dissolved organics pass through the septic tank without any significant treatment [6]. Moreover, these systems have several technical and constructional weaknesses [7]. Thus, septic tank effluent needs further processing in a posttreatment system to meet environmental standards, which increase the cost and complexity of the system. The conventional septic tank removes chemical oxygen demand (COD), biochemical oxygen demand (BOD) total suspended solids (TSS), total Kjeldhal nitrogen (TKN) and helminth eggs to a certain extent [8]. By integrating in-tank baffles, a better contact can be achieved between the wastewater and the active biomass (sludge), leading to increased treatment efficiencies [9]. Many anaerobic
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F.A. Nasr and B. Mikhaeil
modified septic tank systems have been used and tested in different countries [10]. Baffled septic tanks are suitable for all kinds of wastewater, but preferably for those with a high percentage of nonsettleable suspended solids and low COD/BOD ratio. They are ideal for decentralized wastewater treatment because they are simple to build and operate. Hydraulic and organic shock loads have little effect on treatment efficiency. A baffled septic tank consists of several chambers in series. This lets upflow equalized wastewater contact, pass through and be treated by the formed bacteria-rich sludge layer in the bottom of each chamber [11]. In the baffled reactor, the baffles force the incoming wastewater to flow under and over the baffles from the inlet to the outlet of the tank. This improves the contact between anaerobic accumulated sludge and entering wastewater which, in turn, improves the removal of suspended solids due to enmeshment of dissolved anaerobically biodegradable organics [12,13]. There are still important gaps of in our knowledge such as the optimal number of baffles, the optimal hydraulic retention time (HRT) and the potential benefit of an anaerobic filter as polishing stage [14]. Baffled treatment units have shown significantly higher removal efficiencies than the conventional septic tank in terms of total solids (TS), TSS, COD and BOD [15]. Compared with other treatment technologies, an anaerobic modified septic tank occupies a smaller land area, does not need skilled labour to operate it, has much lower operational and maintenance requirements, involves less construction cost, generates much less sludge and releases methane gas, which can be considered a good source of energy if properly recovered [16]. The main objectives of this study were to compare the treatment performance of a conventional septic tank with the baffled type and to try to define the optimal operating conditions.
2.
Figure 1.
Schematic diagram of conventional septic tank.
deep. They had ‘T’ shaped inlet and outlet pipes. Each compartment of the septic tanks had several ports for sludge sampling and other ports on both sides near the top of the tank for the influent feed and effluent discharge (Figure 1). The tanks were covered with 0.6 cm thick polyvinylchloride (PVC). The treatment systems were operated outdoors at ambient climatic conditions in the experimental area of the Water Pollution Research Department at the National Research Centre in Cairo. The systems were fed continuously with domestic wastewater via connection to a neighbouring building. At first no sludge was added to accelerate the growth of the sludge in the models. Dosing pumps were used to feed the septic tanks. The septic tanks were operated at different HRTs and hence different organic loading rates (OLR) in order to arrive at the optimum operating conditions (Table 1). The two-baffle septic tank has the same capacity and a similar design to the conventional one, having two baffle plates installed which divide the tank into three equal compartments in order to achieve for more contact time between domestic wastewater and sludge (Figure 2).
Materials and methods
To accomplish the study’s objectives, laboratory-scale models of a conventional and a two-baffle septic tank were designed and manufactured. Each type tank was made of Perspex material with approximately the same volume of 95 L and dimensions of 64 cm long, 38 cm wide and 39 cm Table 1.
2.1.
Sampling and analytical methods
Twenty-four hour composite samples from the treated effluent at the outlet point of the septic tanks and the raw wastewater were collected and analysed on weekly basis.
Operating conditions of the septic tanks. Operating conditions
HRT (h) Flow rate Q (L/d) HLR (m3 /m3 /day) OLR (kg COD/m3 /day) OLR (kg BOD/m3 /day) Duration period per day
72 31.7 0.33 (0.248–0.393) (0.321)* (0.114–0.187) (0.150)* 210
Note: HLR, hydraulic loading rate. ∗ Average loading rate.
48 47.5 0.5 (0.321–0.535) (0.436)* (0.145–0.240) (0.193)* 210
24 95 1 (0.708–1.050) (0.885)* (0.320–0.445) (0.387)* 210
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Figure 2.
Schematic diagram of two-baffle septic tank.
The measurements covered pH, temperature, total BOD, total chemical oxygen demand (COD), TSS, TS, alkalinity, ammoniacal nitrogen (NH4 -N)+ , TKN, total phosphorus (TP) and fecal coliforms. Periodic measurements of the sludge on a monthly basis covered sludge volume, sludge weight, volatile suspended solids (VSS) and the quantity of accumulated sludge in grams per day during the study period. The analysis was carried out according to the standard method for examination of water and wastewater [17]. Raw wastewater and treated effluent from the two septic tanks were subjected to microbiological investigation using fecal coliforms as an indicator of fecal pollution. Raw wastewater and treated effluent samples were collected in sterile test tubes, covered and sent to the laboratory within minutes. The fecal coliforms were calculated using the multi-tube technique [17]. 3. Results and discussion 3.1. Raw wastewater characteristics The characteristics of the domestic wastewater investigated in this study in terms of COD, BOD, TSS, TKN, NH4 N+ and TP were 960, 450, 295, 71, 26.2 and 4.4 mg/l, Table 2.
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respectively (Table 2). This corresponds to high strength wastewater according to the classification of Metcalf and Eddy [3]. The ratio of COD: BOD for domestic wastewater was around 2.3:1; this provides a good indication that the wastewater can be treated biologically. These values are in agreement with Nasr et al. [18] who stated that, for domestic sewage which is known to be readily biodegradable and can be treated successfully worldwide using a variety of biological treatment methods, the COD/BOD ratio varies from 1.5 to 2. With regard to the nutrient requirements of the wastewater micro-organisms, the average BOD/TKN/TP ratio was 100:16:1. Fecal coliform concentrations recorded an average value of 2.7 × 109 . These values are in agreement with results obtained Nasr et al. [18], but are higher than the results reported by El-Hamouri et al. [19]. The higher value may be attributed to the lower rate of water discharge. 3.2. Performance of the conventional septic tank The performance of the conventional septic tank in this study was investigated using three organic loading rates, ranging between 0.321 and 0.885 kg COD/m3 /day. The results of monitoring the performance of the conventional septic tank at HRT of 24, 48 and 72 h (Table 3) indicated a higher efficiency of 65.3% for COD removal at 72 h descending to 53.4% at 24 h. Corresponding residual COD increased proportionally with the decrease in HRT. Similar results were observed with BOD, which scored 68.4% removal at 72 h HRT and 53.5% at 24 h. TSS percentage removals were 65.3, 58.3 and 55% at HRTs of 72, 48 and 24 h, respectively. These results agree with those obtained by Panswad and Komolmethee [10], who used a full-scale conventional septic tank/anaerobic filter unit with a retention time varying from 22.5 to 90 h and achieved percentage removals of 52.1%, 56% and 53.6% for COD, BOD and TSS at an average retention time of 22.5 h. The results of the present study are also in line with those obtained by Nguyen et al. [14], who obtained average removal efficiencies from 48 to 65% and 44 to 69% in terms of COD and TSS, respectively, depending on the HRT in
Characteristics of raw wastewater investigated in this study. Domestic wastewater
Parameters Temperature pH Total COD Total BOD TSS TS Total phosphorus TKN Ammonia Alkalinity Fecal coliforms
Units ◦C
mg O2 /l mg O2 /l mg/l mg/l mg P/l mg N/l mg N/l mg/l MPN/100ml
Minimum
Maximum
Average
16 5.54 743 341 200 635 3.2 48 20.12 200 9.30E+07
32 7.68 1180 560 412 1150 6.6 100 31.92 340 5.20E+09
27.2 6.6 960 450 295 842 4.44 71 26.2 250 2.7E+09
2340 Table 3.
F.A. Nasr and B. Mikhaeil Average characteristics of the conventional septic tank effluent and percentage removal. 72 h HRT
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pH Temp Alkalinity TSS TS COD BOD TP TKN Ammonia Fecal coliforms
Units ◦C
mg/l mg/l mg/l Mg O2 /l Mg O2 /l mg/l mg/l mg/l MPN/100ml
Average
Study
7.3 27.5 280 103 522 334 142 3.14 52.3 28 3.12E+08
0.23 4.31 20.1 16.1 44.6 33.3 15.9 0.3 6.3 2 3.21E+08
24 h HRT
R%
Average
Study
−12 65.3 38 65.3 68.4 29.3 26.8 −7.1 86
7.4 24 272 115 685 380 164 3 54.4 27.1 8.83E+08
0.34 1.92 10.7 13.7 43.9 43 19.8 0.4 5.3 1.8 6.57E+08
the conventional septic tank. However, the results from the present study are higher than those obtained by Burubai et al. [20], who recorded a COD removal range from 53.1 to 57.3% at HRTs of 24 h and 72 h. The present study results are lower than those obtained by Moussavi et al. [21], who studied the performance of a pilot-scale upflow septic tank for on-site decentralized treatment of residential wastewater at 24 h HRT, and achieved removal of 85%, 77% and 86% for BOD, COD and TSS, respectively, at steady state operation. Since anaerobic digestion takes place in the septic tank, little removal of nitrogen and phosphorus can be expected [22,23]. The results of this study indicated that the removal of phosphorus at HRTs of 72, 48 and 24 h were 29.3%, 26.9% and 25.6%, respectively. These results are higher than those obtained by Mahmoud et al. [24], who attributed the lower phosphorus removal achieved to the relatively low biomass production in anaerobic systems. However, the results are in line with Wanasen [25], who interpreted the phosphorus removal as being utilized for biomass growth, precipitated and entrapped with the digested sludge. The ammonia concentrations at HRTs of 72, 48 and 24 h increased by 7.1%, 5.2% and 4.2%, respectively. The present study results correspond favourably to those obtained by Harrison et al. [26], who obtained residual NH4 -N+ with an average value of 29.3 mg N/l. The removal values of total Kjeldahl nitrogen at the above-mentioned HRTs were 26.8%, 20.8% and 17.7%, respectively. In addition, the results correspond favourably to those obtained by Mahmoud et al. [24] and Dama et al. [27], who attributed the increase in ammonia concentration to degradation of biodegradable nitrogen compounds, and explained the removal of non-biodegradable nitrogen compounds as becoming entrapped in the reactor sludge. In the present study, the alkalinity concentrations at HRTs of 72, 48 and 24 h increased by 12%, 10% and 4%, respectively, and the total solids percentage removals at HRTs of 72, 48 and 24 h were 38%, 27% and 24.6%, respectively. Bacteriological examination of the conventional septic tank effluent revealed a removal efficiency of fecal coliform
R%
Average
Study
R%
−10 58.3 27 56 57 26.9 20.8 −5.2 85
7.5 27.3 240 123 690 412 180 3.14 54.4 26.6 7.60E+08
0.35 0.94 10.8 12.9 46.3 38.9 17.8 0.3 2.8 1.9 2.73E+08
−4 55 24.6 53.4 53.5 25.6 17.7 −4.2 83
less than one log at HRTs of 72, 48 and 24 h, with an average residual value in the final effluent of 108 most probable number (MPN) per 100 ml. This study’s results are in line with those obtained by Harrison et al. [26], who obtained a fecal coliform bacteria count in the effluent of septic tanks ranging from 2.5 × 107 to 1.2 × 108 MPN per 100 ml. One of the mechanisms septic tanks use to remove solids is to make solids settle as sludge at the bottom of the tank [28]. The results indicated that the volume increased progressively with time at all retention times, from 3400 cm3 to 12,500 cm3 , from 2700 cm3 to 10,500 cm3 , and from 1500 cm3 to 8500 cm3 at HRTs of 24, 48 and 72 h, respectively. Sludge weight results followed the same trend from 130 g to 1850 g, from 100 g to 1150 g, and from 25 g to 560 g at HRTs of 24, 48 and 72 h, respectively (Figure 3). The sludge weight per day showed consistent results with those obtained for sludge volume and weight, i.e. from 4.3 to 8.8 g/d, from 3.3 to 4.5 g/d, and from 0.8 to 2.7 g/d at HRTs of 24, 48 and 72 h, respectively. The volatile organic matter content of the sludge decreased over time from 72.1% to 56%, from 70.2% to 54.1%, and from 68.2% to 53.5% at HRTs of 24, 48 and 72 h, respectively. A sludge volume of 12,500 cm3 , 10,500 cm3 and 8500 cm3 was measured at the end of seven months of operation of the conventional
2000
Sludge accumulation (g)
Parameters
48 h HRT
1800
72hHRT
1600
48h HRT
1400
24h HRT
1200 1000 800 600 400 200 0
30
60
90
120 Time (days)
150
180
210
Figure 3. Accumulation rate of sludge over time in conventional septic tank.
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septic tank at HRTs of 24, 48 and 72 h. Assuming that sludge accumulation is proportional to time of operation and that de-sludging should occur at 60% of the septic tank volume as recommended by the US Environmental Protection Agency (USEPA) [5], de-sludging will be required after 32, 38 and 47 months of operation at HRTs of 24, 48 and 72 h, respectively. 3.3. Performance of the two-baffle septic tank The performance of the two-baffle septic tank was investigated using three organic loading rates, ranging between 0.321 and 0.885 kg COD/m3 /day. The results of monitoring the performance of the two baffles septic tank at HRTs of 24, 48 and 72 h as shown in Table 4 indicated a higher efficiency of 74% COD removal at 72 h descending to 57% at 24 h. Corresponding residual COD increases proportionally with the decrease in HRT. Similar results were observed with BOD, which scored 76.5% removal at 72 h HRT and 60% at 24 h. TSS percentage removals were 76%, 72% and 68% at HRTs of 72, 48 and 24 h, respectively. These results agree with those obtained by Kamel and Hegazy [29], who obtained more than 65% reduction in BOD and TSS when operating the baffled septic tank at a HRT of 60.5 h. The present study results were in line with those obtained by Nguyen et al. [11], who obtained average removal efficiencies from 58% to 76% and 61% to 78% in terms of COD and TSS, respectively, depending on the HRT in the baffled septic tank. The present study results indicated that the removal of phosphorus at HRTs of 72, 48 and 24 h were 33.1%, 31% and 29.3%, respectively. These results are in line with results achieved by Nasr et al. [18] and Wanasen [25]. The ammonia concentrations at HRTs of 72, 48 and 24 h increased by 14.2%, 10% and 7.2%, respectively. These results correspond favourably to those obtained by Nguyen et al. [14], who attributed the increase in ammonia concentration in the effluent compared with the concentration of the influent due to the hydrolysis of wastewater occurring in the tank. High nitrogen ammonia concentrations could be an Table 4.
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important reason for not achieving high removal efficiency rates in the tank in some cases [14]. The removal values of total Kjeldahl nitrogen at the above-mentioned HRTs were 31.2%, 26.5% and 21.8%, respectively. These results were higher than those obtained by Koottatep et al. [15]. This is explained by the removal of non-biodegradable nitrogen compounds as they become entrapped in the reactor sludge. The alkalinity concentrations at HRTs of 72, 48 and 24 h increased by 18%, 15% and 7%, respectively. The total solid percentage removals at HRTs of 72, 48 and 24 h were 42.9%, 35% and 31.3%, respectively. Bacteriological examination of the two-baffle septic tank effluent revealed a removal efficiency of fecal coliforms of 95%, 93% and 90% at HRTs of 72, 48 and 24 h, respectively, with average residual values in the final effluent of 108 MPN/100 ml. These results agree with those obtained by Kamel and Hegazy [29]. The results indicated that the volume of the accumulated sludge increased progressively with time at all retention times from 3800 to 13,600 cm3 , from 3080 to 12,600 cm3 , and from 1720 to 10,100 cm3 at HRTs of 24, 48 and 72 h, respectively. Sludge weight results recorded ranged from 162 to 3400 g, from 121 to 1600 g, and from 26 to 850 g at HRTs of 24, 48 and 72 h, respectively (Figure 4). The sludge weight per day showed consistent results with those obtained for sludge volume and weight, i.e. from 5.4 to 16.2 g/d, from 4.1 to 7.6g/d, and from 0.87 to 4.1g/d at HRTs of 24, 48 and 72 h, respectively. The volatile organic matter content of the sludge decreased over time from 72.1% to 56%, from 70.2% to 54%, and from 68.2% to 53.5% at HRTs of 24, 48 and 72 h, respectively. A sludge volume of 13,600 cm3 , 12,600 cm3 and 10,100 cm3 was measured at the end of seven months of operation of the two baffleseptic tank at HRTs of 24, 48 and 72 h. Assuming that sludge accumulation is proportional to operation and that de-sludging should occur when the sludge volume reaches 60% of the septic tank volume as recommended by USEPA [5], de-sludging will be required after 29, 31.5 and 39.5 months at HRTs of 24, 48 and 72 h, respectively.
Average characteristics of the two-baffle septic tank effluent and percentage removal. 72 h HRT
Parameters pH Temperature Alkalinity TSS TS COD BOD TP TKN Ammonia Fecal coliforms
Units ◦C
mg/l mg/l mg/l Mg O2 /l Mg O2 /l mg/l mg/l mg/l MPN/100ml
Average
Study
7.5 27.8 296 71 481 248 106 2.97 49.2 29.9 1.08E+08
0.23 4.43 37 17.1 40 38.7 18.2 0.36 3 1.5 1.02E+08
48 h HRT R%
Average
Study
−18 76 42.9 74 76.5 33.1 31.2 −14.2 95
7.6 24.1 285 77 608 314 134 2.83 50.5 28.3 3.99E+08
0.33 1.97 9.8 10.8 42.7 46.1 19.9 0.47 2.6 1.2 6.79E+08
24 h HRT R%
Average
Study
R%
−15 72 35 64 65 31 26.5 −10 93
7.8 27.2 246 87 629 380 156 2.98 51.7 27.4 4.38E+08
0.33 1.18 11.8 12 46.4 40.1 22.6 0.39 2.3 1.1 4.34E+08
−7 68 31.3 57 60 29.3 21.8 −7.2 90
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F.A. Nasr and B. Mikhaeil
3500 72hHRT
Sludge accumulation (g)
3000 2500
48h HRT 24h HRT
2000 1500 1000 500 0 30
60
90
120 150 Time(days)
180
210
3.4.
Comparison between conventional and two-baffle septic tanks
As can be inferred from its name, the flow direction through the reactor in a baffled septic tank is changed to the vertical flow mode rather than the conventional horizontal flow. This type of operation improves the contact between anaerobic accumulated sludge and entering wastewater, which in turn improves the removal of suspended solids due to enmeshment of dissolved anaerobically biodegradable organics [12,13]. There is no doubt that the number of baffles plays an important role in the treatment process [15,30]. The average results of this study clearly indicate the considerable potential of the baffled septic tank as an alternative to conventional septic tanks for domestic wastewater treatment. Average removal efficiencies from 57% to 74%, from 60% to 76.5%, and from 68% to 76% in terms of COD, BOD and TSS, respectively, could be reached, depending on the HRT in the baffled septic tank. However, the conventional septic tank reactor, under identical working conditions, had average removal efficiencies from 53.4 % to 65.3%, from 53.5% to 68.4%, and from 55% to 65.3% in terms of COD, BOD and TSS, respectively. Figure 5 shows the treated effluent characteristics of both tanks. Comparing the sludge characteristics produced by the conventional and two-baffle septic tanks it is observed that the accumulated sludge volume at each hydraulic retention 500
COD
BOD
TSS
400
mg/l
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Figure 4. Accumulation rate of sludge over time in two-baffle septic tank.
300 200 100 0 72h 72h 48h conventional two-baffles conventional
48h 24h 24h two-baffles conventional two-baffles
Figure 5. COD, BOD and TSS of the treated effluent by conventional and two-baffle septic tanks.
time (24, 48 and 72 h) for the baffled septic tank is higher than corresponding values of the conventional septic tank sludge. The variation in volume ranges from 10 to 15%. Sludge weight shows similar results where it is found to be higher for the two-baffle septic tank than for the conventional one. Similarly the average sludge accumulated per day is higher for the two-baffle septic tank than the conventional one. When comparing the sludge volatile organic matters of the two tank types at each HRT it is found that the average value is almost equal and ranges from 53.5% to 56%. These results are similar to those obtained by Moussavi et al. [21] and lower than those obtained by Halalsheh et al. [31]. The due time for de-sludging of the two-baffle septic tank is 30.5, 31.5 and 39.5 months at HRTs of 24, 48 and 72 h, respectively. These figures are all less than the durations for de-sludging of the conventional septic tank, implying that faster accumulation of sludge takes place in the two-baffle type than in the conventional type. 4.
Conclusions
The average characteristics of raw domestic sewage investigated in this study in terms of COD, BOD and TSS were 960, 450 and 295 mg/l, respectively. This raw wastewater can be categorized of high strength as per a globally recognized classification. Operating the conventional septic tank at HRTs of 24 h and 72 h gave a COD removal rate between 53.4% and 65.3%. The two-baffle septic tank exhibited superior results at the same HRTs with a COD removal rate between 57% and 74%. Operating the conventional septic tank at HRTs of 24 h and 48 h gave close results at the two HRTs. Similarly, operating the two-baffle septic tank at HRTs of 24 h and 48 h also gave close results at the two HRTs, an indication of the feasible selection of the 24 h HRT for optimum performance/operation based on economic advantage. Comparing the treated domestic wastewater quality produced by the two types of septic tanks in terms of physicochemical and biological characteristics, better results were obtained using the two-baffle type an indication of the adequacy of both types for primary treatment. Further processing by a post-treatment system, however, has to be applied to meet environmental standards. Faster accumulation of sludge took place in the twobaffle septic tank compared with the conventional type as the accumulated volume variation was 10–15% in excess to that of the other type. As a result, the frequency of desludging of the two-baffle septic tank was higher than that required for the conventional type, i.e. 29, 31.5 and 39.5 months versus 32, 38 and 47 months at HRTs of 24, 48 and 72 h, respectively. Based on the results achieved, the two-baffle septic tank is considered a viable solution for the on-site decentralized treatment of high strength domestic wastewater especially at rural communities.
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References [1] Les D, Ashantha G. An investigation into the role of site and soil characteristics in on-site sewage treatment. Environ Geol. 2003;44:467–477. [2] He Q, Li J, Liu H, Tang C, de Koning J, Spaniers H. Efficiency of a pilot-scale integrated sludge thickening and digestion reactor in treating low-organic excess sludge. Environ Technol. 2012 Jun;33:1403–1408. [3] Metcalf and Eddy Inc. Wastewater engineering: treatment, disposal and reuse. 4th ed. New York: McGraw-Hill; 2005. [4] Cullimore DR, Viraraghavan T. Microbiological aspects of anaerobic filter treatment of septic tank effluent at low temperatures. Environ Technol. 2008;15:165–173. [5] US Environmental Protection Agency. On-site wastewater treatment systems manual. EPA/625/R-00/008. Washington, DC: Office of Water and Office of Research and Development; 2002. [6] Tchobanoglous G, Burton FL, Stensel HD, editors. Wastewater engineering: treatment and reuse. 4th ed. Metcalf and Eddy Inc. New York: McGraw-Hill; 2003. [7] Coelho AL, do Nascimenio MB, Cavalcanti PF, van Haandel AC. The UASB reactor as an alternative for the septic tank for on-site sewage treatment. Water Sci Technol. 2003;48:221– 226. [8] Andreadakis AD, Christoulas DG. On site filtration and subsurface disposal of domestic sewage. Environ Technol Lett. 1982;3:69–74. [9] Langenhoff AM, Stuckey DC. Treatment of dilute soluble and colloidal wastewater using an anaerobic baffled reactor: effect of low temperature. Water Res. 2000;34:3867– 3875. [10] Panswad T, Komolmethee L. Effects of hydraulic shock loads on small on-site sewage treatment unit. Water Sci Technol. 1997;35:145–152. [11] Nguyen VA, Nga PT, Nguyen HT, Morel A. Improved septic tank and, a promising decentralized wastewater treatment alternative in Vietnam. Paper presented at: GMSARN International Conference on Sustainable Development: Issues and Prospects for GMS; 2006 December 6–7; Hanoi, Vietnam. [12] Zeeman G, Lettinga G. The role of anaerobic digestion of domestic sewage in closing water and nutrient cycle at community level. Water Sci Technol. 1999;39:187–194. [13] Luostarinen SA, Rintala JA. Anaerobic on-site treatment of kitchen waste in combination with black water in UASB-septic tanks at low temperatures. Bioresour Technol. 2007;98:1734–1740. [14] Nguyen VA, Pham TN, Tran HN, Morel A. The potential of decentralized wastewater management for sustainable development – a Vietnamese experience. Paper present at: Water Environmental Federation (WEF) International Conference: Technology; 2005 August 28–31; San Francisco, CA, USA. [15] Koottatep T, Morel A, Sri-Anant W, Schertenleib R. Potential of the anaerobic baffled reactor as decentralized wastewater treatment system in the tropics. Paper presented at: 1st International Conference on Onsite Wastewater Treatment & Recycling; 2004 February 11–13; Perth, Australia. [16] Sabry T. Evaluation of decentralized treatment of sewage employing upflow septic tank/baffled reactor (USBR)
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in developing countries. J Hazard Mater. 2010;174: 500–505. APHA. Standard methods for the examinations of water and wastewater. 21st ed. Washington, DC: American Public Health Association; 2005. Nasr FA, Doma HS, Nassar HF. Treatment of domestic wastewater using an anaerobic baffled reactor followed by a duckweed pond for agricultural purposes. Environmentalist. 2009;29:270–279. El-Hamouri B, Jellal J, Outabiht H, Nebri B, Khallayoune K, Benkerroum A. The performance of a high-rate algal pond in the Moroccan climate. Water Sci Technol. 1995 Dec;31:67– 74. Burubai W, Akor A, Lilly M, Ayawari D. An evaluation of septic tank performance in Bayelsa State, Nigeria. CIGR Ejournal. 2007;IX:1–9. Moussavi G, Kazembeigi F, Farzadkia M. Performance of a pilot scale up-flow septic tank for on-site decentralized treatment of residential wastewater. Process Saf Environ Prot. 2010;88:47–52. Ruiz I, Álvarez JA, Díaz MA, Serrano L, Soto M. Municipal wastewater treatment in an anaerobic digester – constructed wetland system. Environ Technol. 2008;29:1249–1256. Mohapatra DP, Ghangrekar MM, Mitra A, Brar SK. Sewage treatment in integrated system of UASB reactor and duckweed pond and reuse for aquaculture. Environ Technol. 2012;33:1445–1453. Mahmoud M, Tawfik A, Samhan F, El-Gohary F. Sewage treatment using an integrated system consisting of anaerobic hybrid reactor (AHR) and downflow hanging sponge (DHS). Desalin Water Treat. 2009;4:168–176. Wanasen S. Upgrading conventional septic tanks by integrating in-tank baffles [thesis]. EV-03-20. Bangkok, Thailand: Asian Institute of Technology; 2003 Harrison RB, Turner NS, Hoyle JA, Krejsl J, Tone DD, Henry CL, Isaksen PJ, Xue D. Treatment of septic effluent for fecal coliform and nitrogen in coarse-textured soils: use of soil-only and sand filter systems. Water Air Soil Pollut. 2000;124:205–215. Dama P, Bell J, Foxon KM, Brouaert CJ, Huang T, Buckley CA, Naidoo V, Stukey D. Pilot-scale study of an anaerobic baffled reactor for the treatment of domestic wastewater. Paper presented at: 3rd International Conference on Ecological Sanitation; 2005 May 23–26; Durban, South Africa. Crites R, Tchobanoglous G. Small and decentralized wastewater management systems. International edition. Boston, MA: McGraw-Hill; 1998. Kamel MM, Hgazy BE. A septic tank system: on site disposal. J Appl Sci. 2006;6:2269–2274. Barber WP, Stuckey DC. The use of the anaerobic baffled reactor (ABR) for wastewater treatment: a review. Water Res. 1999;33:1559–1578. Halalsheh M, Sawajneh Z, Zubi M, Zeeman G, Lier J, Fayyad M, Lettinga G. Treatment of strong domestic sewage in a 96 m3 UASB reactor operated at ambient temperatures: two-stage versus single-stage reactor. Bioresour Technol. 2005;96:577–585.
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Treatment of domestic wastewater using conventional and baffled septic tanks a
a
Fayza Aly Nasr & Basem Mikhaeil a
Water Pollution Research Department, National Research Centre, Cairo, Egypt Accepted author version posted online: 17 Jan 2013.Published online: 26 Feb 2013.
To cite this article: Fayza Aly Nasr & Basem Mikhaeil (2013) Treatment of domestic wastewater using conventional and baffled septic tanks, Environmental Technology, 34:16, 2337-2343, DOI: 10.1080/09593330.2013.767285 To link to this article: http://dx.doi.org/10.1080/09593330.2013.767285
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Environmental Technology, 2013 Vol. 34, No. 16, 2337–2343, http://dx.doi.org/10.1080/09593330.2013.767285
Treatment of domestic wastewater using conventional and baffled septic tanks Fayza Aly Nasr and Basem Mikhaeil∗ Water Pollution Research Department, National Research Centre, Cairo, Egypt
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(Received 5 January 2012; final version received 13 January 2013 ) The main theme of the study was a comparative study of domestic wastewater treatment using conventional and baffled septic tanks. The septic tanks were fed continuously with domestic wastewater at three different hydraulic retention times (HRTs). The HRTs chosen were 24, 48 and 72 h with corresponding organic loads of 0.321, 0.436 and 0.885 kg chemical oxygen demand (COD) per m3 per day, respectively. The performance of the septic tanks at the three HRTs gave satisfactory results. For the conventional septic tank, COD removal was 53.4%, 56% and 65.3%, at an HRT of 24, 48 and 72 h, respectively, with residual COD of 412, 380 and 334 mg/l, respectively. At HRTs of 72, 48 and 24 h, the following percentages removals were realized for: biochemical oxygen demand (BOD), 68.4%, 57, 53.5%; total suspended solid (TSS), 65.3%, 58.3, 55%; phosphorus, 29.3%, 26.9, 25.6%; total Kjeldahl nitrogen 26.8%, 20.8, 17.7%, respectively. On the contrary, ammonia concentrations increased by 7.1%, 5.2 and 4.2% under the same conditions. Consequently, the results showed that the removal of fecal coliform at all HRTs was less than one log. The two baffled septic tanks exhibited superior results at HRTs of 72, 48 and 24 h. Comparing the treated domestic wastewater quality produced by the two types of septic tanks in terms of physico-chemical and biological characteristics, better results were obtained using the two baffles type. Keywords: domestic; wastewater; treatment; conventional; baffled; septic; de-sludging.
1. Introduction Its considerable advantages mean that the simple septic tank system is the most commonly known primary treatment method for onsite wastewater treatment. Septic tanks remove most settleable solids and function as an anaerobic bioreactor that promotes partial digestion of organic matter. The main cause of their failure is the unsuitability of the soil and the site characteristics [1]. The system is inexpensive, and simple to operate and maintain, although sludge may cause an odour problem if left untreated for a long time [2]. Conventional onsite wastewater treatment systems are not effective in removing nitrate and phosphorus compounds and in reducing pathogenic organisms [3,4]. Septic tanks may be used alone or in combination with other processes to treat raw wastewater before it is discharged to a subsurface infiltration system. The tank provides primary treatment by creating quiescent conditions inside a covered, watertight rectangular, oval or cylindrical vessel, which is typically buried. In addition to primary treatment, the septic tank can reduce the sludge and scum volumes by as much as 40%. It also conditions the wastewater by hydrolysing organic molecules for subsequent treatment in the soil or by other unit processes. The outlet connections (e.g. a sanitary ‘tee’ fitting) retain the sludge and scum layers in the tank and draw effluent only from the clarified zone between the sludge and scum layers.
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
Septic tanks are used in nearly all onsite systems regardless of the daily wastewater flow rate or strength. The tanks provide suspended solids removal, solids storage and digestion [5]. Three zones are present in a septic tank: a scum layer which forms a crust on the surface of the tank liquor; the wastewater from which solids deposit; and a bottom sludge layer of deposited material. The organic matter in the tank may undergo anaerobic digestion. The degree of digestion depends on the tank size, frequency of cleaning and temperature. The capacity of the tank required is governed by the number of people it serves and desludging interval. Although a fraction of particulate solids are removed by flotation and sedimentation, nearly all entering dissolved organics pass through the septic tank without any significant treatment [6]. Moreover, these systems have several technical and constructional weaknesses [7]. Thus, septic tank effluent needs further processing in a posttreatment system to meet environmental standards, which increase the cost and complexity of the system. The conventional septic tank removes chemical oxygen demand (COD), biochemical oxygen demand (BOD) total suspended solids (TSS), total Kjeldhal nitrogen (TKN) and helminth eggs to a certain extent [8]. By integrating in-tank baffles, a better contact can be achieved between the wastewater and the active biomass (sludge), leading to increased treatment efficiencies [9]. Many anaerobic
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F.A. Nasr and B. Mikhaeil
modified septic tank systems have been used and tested in different countries [10]. Baffled septic tanks are suitable for all kinds of wastewater, but preferably for those with a high percentage of nonsettleable suspended solids and low COD/BOD ratio. They are ideal for decentralized wastewater treatment because they are simple to build and operate. Hydraulic and organic shock loads have little effect on treatment efficiency. A baffled septic tank consists of several chambers in series. This lets upflow equalized wastewater contact, pass through and be treated by the formed bacteria-rich sludge layer in the bottom of each chamber [11]. In the baffled reactor, the baffles force the incoming wastewater to flow under and over the baffles from the inlet to the outlet of the tank. This improves the contact between anaerobic accumulated sludge and entering wastewater which, in turn, improves the removal of suspended solids due to enmeshment of dissolved anaerobically biodegradable organics [12,13]. There are still important gaps of in our knowledge such as the optimal number of baffles, the optimal hydraulic retention time (HRT) and the potential benefit of an anaerobic filter as polishing stage [14]. Baffled treatment units have shown significantly higher removal efficiencies than the conventional septic tank in terms of total solids (TS), TSS, COD and BOD [15]. Compared with other treatment technologies, an anaerobic modified septic tank occupies a smaller land area, does not need skilled labour to operate it, has much lower operational and maintenance requirements, involves less construction cost, generates much less sludge and releases methane gas, which can be considered a good source of energy if properly recovered [16]. The main objectives of this study were to compare the treatment performance of a conventional septic tank with the baffled type and to try to define the optimal operating conditions.
2.
Figure 1.
Schematic diagram of conventional septic tank.
deep. They had ‘T’ shaped inlet and outlet pipes. Each compartment of the septic tanks had several ports for sludge sampling and other ports on both sides near the top of the tank for the influent feed and effluent discharge (Figure 1). The tanks were covered with 0.6 cm thick polyvinylchloride (PVC). The treatment systems were operated outdoors at ambient climatic conditions in the experimental area of the Water Pollution Research Department at the National Research Centre in Cairo. The systems were fed continuously with domestic wastewater via connection to a neighbouring building. At first no sludge was added to accelerate the growth of the sludge in the models. Dosing pumps were used to feed the septic tanks. The septic tanks were operated at different HRTs and hence different organic loading rates (OLR) in order to arrive at the optimum operating conditions (Table 1). The two-baffle septic tank has the same capacity and a similar design to the conventional one, having two baffle plates installed which divide the tank into three equal compartments in order to achieve for more contact time between domestic wastewater and sludge (Figure 2).
Materials and methods
To accomplish the study’s objectives, laboratory-scale models of a conventional and a two-baffle septic tank were designed and manufactured. Each type tank was made of Perspex material with approximately the same volume of 95 L and dimensions of 64 cm long, 38 cm wide and 39 cm Table 1.
2.1.
Sampling and analytical methods
Twenty-four hour composite samples from the treated effluent at the outlet point of the septic tanks and the raw wastewater were collected and analysed on weekly basis.
Operating conditions of the septic tanks. Operating conditions
HRT (h) Flow rate Q (L/d) HLR (m3 /m3 /day) OLR (kg COD/m3 /day) OLR (kg BOD/m3 /day) Duration period per day
72 31.7 0.33 (0.248–0.393) (0.321)* (0.114–0.187) (0.150)* 210
Note: HLR, hydraulic loading rate. ∗ Average loading rate.
48 47.5 0.5 (0.321–0.535) (0.436)* (0.145–0.240) (0.193)* 210
24 95 1 (0.708–1.050) (0.885)* (0.320–0.445) (0.387)* 210
Environmental Technology
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Figure 2.
Schematic diagram of two-baffle septic tank.
The measurements covered pH, temperature, total BOD, total chemical oxygen demand (COD), TSS, TS, alkalinity, ammoniacal nitrogen (NH4 -N)+ , TKN, total phosphorus (TP) and fecal coliforms. Periodic measurements of the sludge on a monthly basis covered sludge volume, sludge weight, volatile suspended solids (VSS) and the quantity of accumulated sludge in grams per day during the study period. The analysis was carried out according to the standard method for examination of water and wastewater [17]. Raw wastewater and treated effluent from the two septic tanks were subjected to microbiological investigation using fecal coliforms as an indicator of fecal pollution. Raw wastewater and treated effluent samples were collected in sterile test tubes, covered and sent to the laboratory within minutes. The fecal coliforms were calculated using the multi-tube technique [17]. 3. Results and discussion 3.1. Raw wastewater characteristics The characteristics of the domestic wastewater investigated in this study in terms of COD, BOD, TSS, TKN, NH4 N+ and TP were 960, 450, 295, 71, 26.2 and 4.4 mg/l, Table 2.
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respectively (Table 2). This corresponds to high strength wastewater according to the classification of Metcalf and Eddy [3]. The ratio of COD: BOD for domestic wastewater was around 2.3:1; this provides a good indication that the wastewater can be treated biologically. These values are in agreement with Nasr et al. [18] who stated that, for domestic sewage which is known to be readily biodegradable and can be treated successfully worldwide using a variety of biological treatment methods, the COD/BOD ratio varies from 1.5 to 2. With regard to the nutrient requirements of the wastewater micro-organisms, the average BOD/TKN/TP ratio was 100:16:1. Fecal coliform concentrations recorded an average value of 2.7 × 109 . These values are in agreement with results obtained Nasr et al. [18], but are higher than the results reported by El-Hamouri et al. [19]. The higher value may be attributed to the lower rate of water discharge. 3.2. Performance of the conventional septic tank The performance of the conventional septic tank in this study was investigated using three organic loading rates, ranging between 0.321 and 0.885 kg COD/m3 /day. The results of monitoring the performance of the conventional septic tank at HRT of 24, 48 and 72 h (Table 3) indicated a higher efficiency of 65.3% for COD removal at 72 h descending to 53.4% at 24 h. Corresponding residual COD increased proportionally with the decrease in HRT. Similar results were observed with BOD, which scored 68.4% removal at 72 h HRT and 53.5% at 24 h. TSS percentage removals were 65.3, 58.3 and 55% at HRTs of 72, 48 and 24 h, respectively. These results agree with those obtained by Panswad and Komolmethee [10], who used a full-scale conventional septic tank/anaerobic filter unit with a retention time varying from 22.5 to 90 h and achieved percentage removals of 52.1%, 56% and 53.6% for COD, BOD and TSS at an average retention time of 22.5 h. The results of the present study are also in line with those obtained by Nguyen et al. [14], who obtained average removal efficiencies from 48 to 65% and 44 to 69% in terms of COD and TSS, respectively, depending on the HRT in
Characteristics of raw wastewater investigated in this study. Domestic wastewater
Parameters Temperature pH Total COD Total BOD TSS TS Total phosphorus TKN Ammonia Alkalinity Fecal coliforms
Units ◦C
mg O2 /l mg O2 /l mg/l mg/l mg P/l mg N/l mg N/l mg/l MPN/100ml
Minimum
Maximum
Average
16 5.54 743 341 200 635 3.2 48 20.12 200 9.30E+07
32 7.68 1180 560 412 1150 6.6 100 31.92 340 5.20E+09
27.2 6.6 960 450 295 842 4.44 71 26.2 250 2.7E+09
2340 Table 3.
F.A. Nasr and B. Mikhaeil Average characteristics of the conventional septic tank effluent and percentage removal. 72 h HRT
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pH Temp Alkalinity TSS TS COD BOD TP TKN Ammonia Fecal coliforms
Units ◦C
mg/l mg/l mg/l Mg O2 /l Mg O2 /l mg/l mg/l mg/l MPN/100ml
Average
Study
7.3 27.5 280 103 522 334 142 3.14 52.3 28 3.12E+08
0.23 4.31 20.1 16.1 44.6 33.3 15.9 0.3 6.3 2 3.21E+08
24 h HRT
R%
Average
Study
−12 65.3 38 65.3 68.4 29.3 26.8 −7.1 86
7.4 24 272 115 685 380 164 3 54.4 27.1 8.83E+08
0.34 1.92 10.7 13.7 43.9 43 19.8 0.4 5.3 1.8 6.57E+08
the conventional septic tank. However, the results from the present study are higher than those obtained by Burubai et al. [20], who recorded a COD removal range from 53.1 to 57.3% at HRTs of 24 h and 72 h. The present study results are lower than those obtained by Moussavi et al. [21], who studied the performance of a pilot-scale upflow septic tank for on-site decentralized treatment of residential wastewater at 24 h HRT, and achieved removal of 85%, 77% and 86% for BOD, COD and TSS, respectively, at steady state operation. Since anaerobic digestion takes place in the septic tank, little removal of nitrogen and phosphorus can be expected [22,23]. The results of this study indicated that the removal of phosphorus at HRTs of 72, 48 and 24 h were 29.3%, 26.9% and 25.6%, respectively. These results are higher than those obtained by Mahmoud et al. [24], who attributed the lower phosphorus removal achieved to the relatively low biomass production in anaerobic systems. However, the results are in line with Wanasen [25], who interpreted the phosphorus removal as being utilized for biomass growth, precipitated and entrapped with the digested sludge. The ammonia concentrations at HRTs of 72, 48 and 24 h increased by 7.1%, 5.2% and 4.2%, respectively. The present study results correspond favourably to those obtained by Harrison et al. [26], who obtained residual NH4 -N+ with an average value of 29.3 mg N/l. The removal values of total Kjeldahl nitrogen at the above-mentioned HRTs were 26.8%, 20.8% and 17.7%, respectively. In addition, the results correspond favourably to those obtained by Mahmoud et al. [24] and Dama et al. [27], who attributed the increase in ammonia concentration to degradation of biodegradable nitrogen compounds, and explained the removal of non-biodegradable nitrogen compounds as becoming entrapped in the reactor sludge. In the present study, the alkalinity concentrations at HRTs of 72, 48 and 24 h increased by 12%, 10% and 4%, respectively, and the total solids percentage removals at HRTs of 72, 48 and 24 h were 38%, 27% and 24.6%, respectively. Bacteriological examination of the conventional septic tank effluent revealed a removal efficiency of fecal coliform
R%
Average
Study
R%
−10 58.3 27 56 57 26.9 20.8 −5.2 85
7.5 27.3 240 123 690 412 180 3.14 54.4 26.6 7.60E+08
0.35 0.94 10.8 12.9 46.3 38.9 17.8 0.3 2.8 1.9 2.73E+08
−4 55 24.6 53.4 53.5 25.6 17.7 −4.2 83
less than one log at HRTs of 72, 48 and 24 h, with an average residual value in the final effluent of 108 most probable number (MPN) per 100 ml. This study’s results are in line with those obtained by Harrison et al. [26], who obtained a fecal coliform bacteria count in the effluent of septic tanks ranging from 2.5 × 107 to 1.2 × 108 MPN per 100 ml. One of the mechanisms septic tanks use to remove solids is to make solids settle as sludge at the bottom of the tank [28]. The results indicated that the volume increased progressively with time at all retention times, from 3400 cm3 to 12,500 cm3 , from 2700 cm3 to 10,500 cm3 , and from 1500 cm3 to 8500 cm3 at HRTs of 24, 48 and 72 h, respectively. Sludge weight results followed the same trend from 130 g to 1850 g, from 100 g to 1150 g, and from 25 g to 560 g at HRTs of 24, 48 and 72 h, respectively (Figure 3). The sludge weight per day showed consistent results with those obtained for sludge volume and weight, i.e. from 4.3 to 8.8 g/d, from 3.3 to 4.5 g/d, and from 0.8 to 2.7 g/d at HRTs of 24, 48 and 72 h, respectively. The volatile organic matter content of the sludge decreased over time from 72.1% to 56%, from 70.2% to 54.1%, and from 68.2% to 53.5% at HRTs of 24, 48 and 72 h, respectively. A sludge volume of 12,500 cm3 , 10,500 cm3 and 8500 cm3 was measured at the end of seven months of operation of the conventional
2000
Sludge accumulation (g)
Parameters
48 h HRT
1800
72hHRT
1600
48h HRT
1400
24h HRT
1200 1000 800 600 400 200 0
30
60
90
120 Time (days)
150
180
210
Figure 3. Accumulation rate of sludge over time in conventional septic tank.
Environmental Technology
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septic tank at HRTs of 24, 48 and 72 h. Assuming that sludge accumulation is proportional to time of operation and that de-sludging should occur at 60% of the septic tank volume as recommended by the US Environmental Protection Agency (USEPA) [5], de-sludging will be required after 32, 38 and 47 months of operation at HRTs of 24, 48 and 72 h, respectively. 3.3. Performance of the two-baffle septic tank The performance of the two-baffle septic tank was investigated using three organic loading rates, ranging between 0.321 and 0.885 kg COD/m3 /day. The results of monitoring the performance of the two baffles septic tank at HRTs of 24, 48 and 72 h as shown in Table 4 indicated a higher efficiency of 74% COD removal at 72 h descending to 57% at 24 h. Corresponding residual COD increases proportionally with the decrease in HRT. Similar results were observed with BOD, which scored 76.5% removal at 72 h HRT and 60% at 24 h. TSS percentage removals were 76%, 72% and 68% at HRTs of 72, 48 and 24 h, respectively. These results agree with those obtained by Kamel and Hegazy [29], who obtained more than 65% reduction in BOD and TSS when operating the baffled septic tank at a HRT of 60.5 h. The present study results were in line with those obtained by Nguyen et al. [11], who obtained average removal efficiencies from 58% to 76% and 61% to 78% in terms of COD and TSS, respectively, depending on the HRT in the baffled septic tank. The present study results indicated that the removal of phosphorus at HRTs of 72, 48 and 24 h were 33.1%, 31% and 29.3%, respectively. These results are in line with results achieved by Nasr et al. [18] and Wanasen [25]. The ammonia concentrations at HRTs of 72, 48 and 24 h increased by 14.2%, 10% and 7.2%, respectively. These results correspond favourably to those obtained by Nguyen et al. [14], who attributed the increase in ammonia concentration in the effluent compared with the concentration of the influent due to the hydrolysis of wastewater occurring in the tank. High nitrogen ammonia concentrations could be an Table 4.
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important reason for not achieving high removal efficiency rates in the tank in some cases [14]. The removal values of total Kjeldahl nitrogen at the above-mentioned HRTs were 31.2%, 26.5% and 21.8%, respectively. These results were higher than those obtained by Koottatep et al. [15]. This is explained by the removal of non-biodegradable nitrogen compounds as they become entrapped in the reactor sludge. The alkalinity concentrations at HRTs of 72, 48 and 24 h increased by 18%, 15% and 7%, respectively. The total solid percentage removals at HRTs of 72, 48 and 24 h were 42.9%, 35% and 31.3%, respectively. Bacteriological examination of the two-baffle septic tank effluent revealed a removal efficiency of fecal coliforms of 95%, 93% and 90% at HRTs of 72, 48 and 24 h, respectively, with average residual values in the final effluent of 108 MPN/100 ml. These results agree with those obtained by Kamel and Hegazy [29]. The results indicated that the volume of the accumulated sludge increased progressively with time at all retention times from 3800 to 13,600 cm3 , from 3080 to 12,600 cm3 , and from 1720 to 10,100 cm3 at HRTs of 24, 48 and 72 h, respectively. Sludge weight results recorded ranged from 162 to 3400 g, from 121 to 1600 g, and from 26 to 850 g at HRTs of 24, 48 and 72 h, respectively (Figure 4). The sludge weight per day showed consistent results with those obtained for sludge volume and weight, i.e. from 5.4 to 16.2 g/d, from 4.1 to 7.6g/d, and from 0.87 to 4.1g/d at HRTs of 24, 48 and 72 h, respectively. The volatile organic matter content of the sludge decreased over time from 72.1% to 56%, from 70.2% to 54%, and from 68.2% to 53.5% at HRTs of 24, 48 and 72 h, respectively. A sludge volume of 13,600 cm3 , 12,600 cm3 and 10,100 cm3 was measured at the end of seven months of operation of the two baffleseptic tank at HRTs of 24, 48 and 72 h. Assuming that sludge accumulation is proportional to operation and that de-sludging should occur when the sludge volume reaches 60% of the septic tank volume as recommended by USEPA [5], de-sludging will be required after 29, 31.5 and 39.5 months at HRTs of 24, 48 and 72 h, respectively.
Average characteristics of the two-baffle septic tank effluent and percentage removal. 72 h HRT
Parameters pH Temperature Alkalinity TSS TS COD BOD TP TKN Ammonia Fecal coliforms
Units ◦C
mg/l mg/l mg/l Mg O2 /l Mg O2 /l mg/l mg/l mg/l MPN/100ml
Average
Study
7.5 27.8 296 71 481 248 106 2.97 49.2 29.9 1.08E+08
0.23 4.43 37 17.1 40 38.7 18.2 0.36 3 1.5 1.02E+08
48 h HRT R%
Average
Study
−18 76 42.9 74 76.5 33.1 31.2 −14.2 95
7.6 24.1 285 77 608 314 134 2.83 50.5 28.3 3.99E+08
0.33 1.97 9.8 10.8 42.7 46.1 19.9 0.47 2.6 1.2 6.79E+08
24 h HRT R%
Average
Study
R%
−15 72 35 64 65 31 26.5 −10 93
7.8 27.2 246 87 629 380 156 2.98 51.7 27.4 4.38E+08
0.33 1.18 11.8 12 46.4 40.1 22.6 0.39 2.3 1.1 4.34E+08
−7 68 31.3 57 60 29.3 21.8 −7.2 90
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F.A. Nasr and B. Mikhaeil
3500 72hHRT
Sludge accumulation (g)
3000 2500
48h HRT 24h HRT
2000 1500 1000 500 0 30
60
90
120 150 Time(days)
180
210
3.4.
Comparison between conventional and two-baffle septic tanks
As can be inferred from its name, the flow direction through the reactor in a baffled septic tank is changed to the vertical flow mode rather than the conventional horizontal flow. This type of operation improves the contact between anaerobic accumulated sludge and entering wastewater, which in turn improves the removal of suspended solids due to enmeshment of dissolved anaerobically biodegradable organics [12,13]. There is no doubt that the number of baffles plays an important role in the treatment process [15,30]. The average results of this study clearly indicate the considerable potential of the baffled septic tank as an alternative to conventional septic tanks for domestic wastewater treatment. Average removal efficiencies from 57% to 74%, from 60% to 76.5%, and from 68% to 76% in terms of COD, BOD and TSS, respectively, could be reached, depending on the HRT in the baffled septic tank. However, the conventional septic tank reactor, under identical working conditions, had average removal efficiencies from 53.4 % to 65.3%, from 53.5% to 68.4%, and from 55% to 65.3% in terms of COD, BOD and TSS, respectively. Figure 5 shows the treated effluent characteristics of both tanks. Comparing the sludge characteristics produced by the conventional and two-baffle septic tanks it is observed that the accumulated sludge volume at each hydraulic retention 500
COD
BOD
TSS
400
mg/l
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Figure 4. Accumulation rate of sludge over time in two-baffle septic tank.
300 200 100 0 72h 72h 48h conventional two-baffles conventional
48h 24h 24h two-baffles conventional two-baffles
Figure 5. COD, BOD and TSS of the treated effluent by conventional and two-baffle septic tanks.
time (24, 48 and 72 h) for the baffled septic tank is higher than corresponding values of the conventional septic tank sludge. The variation in volume ranges from 10 to 15%. Sludge weight shows similar results where it is found to be higher for the two-baffle septic tank than for the conventional one. Similarly the average sludge accumulated per day is higher for the two-baffle septic tank than the conventional one. When comparing the sludge volatile organic matters of the two tank types at each HRT it is found that the average value is almost equal and ranges from 53.5% to 56%. These results are similar to those obtained by Moussavi et al. [21] and lower than those obtained by Halalsheh et al. [31]. The due time for de-sludging of the two-baffle septic tank is 30.5, 31.5 and 39.5 months at HRTs of 24, 48 and 72 h, respectively. These figures are all less than the durations for de-sludging of the conventional septic tank, implying that faster accumulation of sludge takes place in the two-baffle type than in the conventional type. 4.
Conclusions
The average characteristics of raw domestic sewage investigated in this study in terms of COD, BOD and TSS were 960, 450 and 295 mg/l, respectively. This raw wastewater can be categorized of high strength as per a globally recognized classification. Operating the conventional septic tank at HRTs of 24 h and 72 h gave a COD removal rate between 53.4% and 65.3%. The two-baffle septic tank exhibited superior results at the same HRTs with a COD removal rate between 57% and 74%. Operating the conventional septic tank at HRTs of 24 h and 48 h gave close results at the two HRTs. Similarly, operating the two-baffle septic tank at HRTs of 24 h and 48 h also gave close results at the two HRTs, an indication of the feasible selection of the 24 h HRT for optimum performance/operation based on economic advantage. Comparing the treated domestic wastewater quality produced by the two types of septic tanks in terms of physicochemical and biological characteristics, better results were obtained using the two-baffle type an indication of the adequacy of both types for primary treatment. Further processing by a post-treatment system, however, has to be applied to meet environmental standards. Faster accumulation of sludge took place in the twobaffle septic tank compared with the conventional type as the accumulated volume variation was 10–15% in excess to that of the other type. As a result, the frequency of desludging of the two-baffle septic tank was higher than that required for the conventional type, i.e. 29, 31.5 and 39.5 months versus 32, 38 and 47 months at HRTs of 24, 48 and 72 h, respectively. Based on the results achieved, the two-baffle septic tank is considered a viable solution for the on-site decentralized treatment of high strength domestic wastewater especially at rural communities.
Environmental Technology
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