الفهرس 
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Abstract
The simple septic tank system is the most commonly known primary treatment method for onsite wastewater treatment because of its considerable advantages. Septic tanks remove most settleable solids and function as an anaerobic bioreactor that promotes partial digestion of organic matter. Their main cause of failure is the unsuitability of the soil and the site characteristics. This system is inexpensive and simple to operate and maintain. Yet, sludge may cause an odor problem if kept untreated for a long time. Septic tanks are used in nearly all onsite systems regardless of daily wastewater flow rate or strength. The conventional onsite wastewater treatment systems using septic tank are not effective in removing nitrate and phosphorus compounds and reducing pathogenic organisms. As such, these systems can be used prior to further treatment and disposal. 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 percent, and it conditions the wastewater by hydrolyzing 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. 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 persons it serves and desludging interval. The conventional septic tank removes COD, BOD, TSS, TKN and fecal coliform to a certain extent.
By integrating in-tank baffles, a better contact can be achieved between the wastewater and the active biomass (sludge), leading to increased treatment efficiencies. The baffled septic tanks are suitable for all kinds of wastewater, preferably for those with a high percentage of non-settleable 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 up-flow equalized wastewater contact, pass through and be treated anaerobically by the formed bacteria-rich sludge layer in the bottom of each chamber.
There are still important gaps of knowledge such as the optimal number of baffles, the optimal hydraulic retention time, the potential benefit of an anaerobic filter as polishing stage. Anaerobic modified septic tank, compared to other treatment technologies: occupies smaller land area, does not need skilled labor to operate, has much less operation 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.
4.2 Objective
The objective of this study is to compare the performance of the conventional septic tank system for domestic wastewater treatment with performance of the modified septic tank and try to define their optimal operating conditions.
4.3 materials and methods
This study investigates the treatment performance of the conventional and modified septic tanks for treatment of domestic wastewater. To accomplish the objectives of this study, four laboratory scale septic tank models of conventional, single baffle, two baffles and two baffles with packing material types were designed and manufactured. The treatment systems were operated outdoor at ambient climatic conditions in the experimental area of the Water Pollution Research Department of the National Research Center. The systems were fed continuously with domestic wastewater via connection to a neighboring building. At the beginning no sludge was added to accelerate the growth of the sludge in the models. Dosing pumps were used for feeding septic tanks. Performance of the four septic tanks in this study was investigated using three organic loading rates, ranging between 0.321 and 0.885 kg COD/m3/day.
4.4 Raw wastewater characteristics
Characteristics of domestic wastewater investigated in this study in terms of COD, BOD, TSS, TKN, ammonia and TP were 962, 450, 296, 71.4, 26.2 and 4.4 mg/l, respectively. This corresponds to high strength wastewater according to the world wide classification. 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. 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 2.29 × 109.
4.5 performances of the investigated septic tanks
Four types septic tank of the same volume were investigated in this study using three organic loading rates; 0.321, 0.436 and 0.885 Kg COD/m3/day corresponding to HRT of 72h, 48h and 24h, respectively. At each HRT it was observed that the type of the tank affects the percentage removal of the pollutants such that the packed type returns best results and the conventional type is the least.
Comparing the performance of conventional, single baffle, two baffles and packed type septic tank at 24h HRT in terms of COD percentage removals were 53.4, 55.3, 57 and 59.4%. Corresponding residual values were 412, 396, 380 and 359, respectively. The BOD percentage removals were 53.5, 56, 60 and 63.5%. Corresponding residual values were 180, 170, 156 and 141, respectively. The TSS percentage removals were 55, 65, 68 and 71 %. Corresponding residual values were 123, 96, 87 and 78 respectively.
The results at 48h HRT of the performance of the four types septic tanks in the order described were; COD percentage removals of 56, 60, 64 and 68%. Corresponding residual values were 380, 350, 314 and 279, respectively. The BOD percentage removals were 57, 62, 65 and 69%. Corresponding residual values were 164, 148, 134 and 120, respectively. The TSS percentage removals were 58.3, 69, 72 and 75 % respectively. Corresponding residual values were 115, 85, 77 and 69, respectively.
At 72h HRT the results obtained were; COD percentage removals of 65.3, 72.3, 74 and 77%. Corresponding residual values were 334, 266, 248 and 221, respectively. The BOD percentage removals were 68.4, 74.4, 76.5 and 79%. Corresponding residual values were 142, 115, 106 and 94, respectively. The TSS percentage removals were 65.3, 73, 76 and 78 % respectively. Corresponding residual values were 103, 79, 71 and 65, respectively. It can be observed that the percentage removal of pollutants for each type septic tank improves in direct proportion to the HRT.
The nitrogen and phosphorous removal was found to be dependent on the type of the septic tank and the HRT. It ranged between a minimum and maximum value of 17.7 and 26.8% for nitrogen, 25.6 and 29.3 for phosphorous using conventional septic tank. Corresponding values for packed type septic tank were 22.8 and 34% for nitrogen, 31.2 and 36% for phosphorous. These small removal values of nitrogen and phosphorous are due to the anaerobic digestion which takes place in the septic tank. The fecal coliform removal values were observed to be affected by the same parameters; septic tank type and HRT, however the highest percentage removal reached only one log.
Comparing the sludge characteristics produced by the investigated septic tanks it was observed that the accumulated volume and weight depend on the septic tank type and HRT. When comparing the sludge volatile organic matters of the septic tanks at each HRT it was found that average value is almost equal. Also it was observed that the due time for desludging is directly proportional to the HRT. For example, the conventional type septic tank requires to be desludged after 28, 33 and 40 months at HRT 24h, 48h and 72h, respectively. The due times for desludging of single baffle septic tank were 26, 30.5 and 38 months at HRT 24h, 48h and 72h, respectively. Simillary, results obtained for two baffles septic tank at the same HRT were 25.5, 28.5 and 34.5 months, respectively. The packed type septic tank due time for desludging at the same HRT were 26, 29.5 and 36.5 months, respectively.
4.6 Conclusion
• The average characteristics of raw domestic sewage investigated in this study in terms of COD, BOD, TSS were 962, 450, 296 mg/l. This raw wastewater can be categorized of high strength as per world-recognized classification.
• The treated domestic wastewater quality produced by the four types septic tanks in terms of physico-chemical and biological characteristics proved to be satisfactory. Comparison wise, better results were obtained using the packed type septic tank for primary treatment. Further processing by a post treatment system, however, has to be applied to meet environmental standards.
• The average removal of pollutants for each type of septic tank in terms of COD, BOD and TSS improves in direct proportion to HRT.
• The maximum removal of nitrogen and phosphorous; 34% and 36%, respectively are small values due to the anaerobic digestion which takes place in the septic tank.
• The fecal coliform removal values were observed to be affected by type of the septic tank and the HRT, however the highest percentage removal reached only one log.
• At each HRT it was observed that the type of the tank affects the percentage removal of the pollutants such that the packed type returns best results and the conventional type is the least.
• Operating the four types septic tanks at HRT of 24h and 48h, each type gave close results at the two HRT, an indication of feasible selection of the 24h HRT for optimum performance /operation based on economic advantage.
• The accumulated sludge volume and weight depend on the septic tank type and HRT. The average percentage of sludge volatile organic matters is almost equal. The due time for desludging is directly proportional to the HRT. At each HRT the due time for desludging tank is in the order of conventional > single baffle > packed type > two baffles septic tank
• Based on achieved results, either the two baffles or the packed septic tank is considered a viable solution for the on-site decentralized treatment of high strength domestic wastewater especially at rural communities.