INTEGRATED SETTLER AND ANAEROBIC FILTER BASED ONSITE SEWAGE TREATMENT SYSTEMS

Abstract

In most of the developing countries, the rapid population growth in association with the developmental works has put the water resources under a great stress while increasing the generation of wastewater. Due to financial limitations, it is not practically feasible to provide a sewerage system for the collection of domestic wastewater to each and every household of the sparsely populated areas. The conventional septic tank (CST) with soak pit/soil absorption area is considered to be the most suitable low-cost option for the on-site treatment of domestic wastewater. The National Census of India 2011 revealed that a total of 38.2% of the urban and 14.7% of the rural population rely on the CST. But, on the other hand, there are some significant drawbacks associated with the CST, of which the most important one is its low treatment efficiency. The widespread application of the CST for on-site treatment of domestic wastewater has been significantly contributing to the escalating environmental pollution. Therefore, it is very important to work towards developing and implementing more efficient on-site domestic wastewater treatment systems for the protection of surrounding environment. The present work is an attempt in the direction of developing a more effective and low-cost on-site wastewater treatment system. To improve the performance of the conventional on-site system (septic tank with soakage pit), it was contemplated that either the existing septic tank itself be modified or an effective post-treatment system be utilized. Hence, the present study was carried out in this direction. The study was conducted in four phases. The first phase dealt with identification of the current status in terms of effluent quality of the existing septic tank in the actual field conditions. In the second phase, performance evaluation of the lab-scale CST was done while identifying the possible modifications to improve its treatment efficiency by taking the hydrodynamics into consideration. The third phase was carried out to identify the efficacy of the system developed after modifications in the CST on the basis of its performance in the actual field. In the fourth phase, the effect of applying various post-treatment systems to facilitate recycling and reuse of the treated wastewater was identified. The phase-1 study revealed that the pollutant concentrations in the septic-tank effluent were very high in terms of organic matter and nutrients while containing significant amount of coliforms and pathogenic microorganisms. This was attributed mainly to the poor state of operation and maintenance with lack of desludging of the septic tank. The overall sanitation scenario could be termed as alarmingly poor, which required immediate attention and Integrated Settler and Anaerobic Filter Based Onsite Sewage Treatment Systems Department of Civil Engineering, IIT Roorkee iv effective management of the on-site domestic wastewater systems to safeguard the aquatic environment and public health. The phase-2 was carried out with the main aim of identifying some low-cost approaches to enhance the treatment efficiency of the CST. Therefore, three on-site systems were designed and studied in three different stages. The stage-1 was related to the performance of CST used as a control system. It was observed that the unsatisfactory performance of the CST was mainly due to its poor design and the horizontal flow regime. The stage-2 and 3 were associated with the performance evaluation of the systems developed after modifications in the CST. The stage-2 was related to the modification in structure and operation of the CST (modified septic tank), while stage-3 involved the modified septic tank followed by post-treatment option. In stage-2, simple changes were made in the design of the CST by incorporating a partition wall in the middle of the tank and using a longer inlet pipe; while in stage-3, an upflow anaerobic filter was also introduced as a post-treatment measure for the modified septic tank effluent, which was accommodated within the same unit. Thus, the final design of the system turned it into a two-stage treatment system. The physical properties of the supporting media also play very a important role during the treatment of wastewater as these change the hydrodynamic characteristics of the system. Therefore, to examine the effect of different media, another stage of the study (stage 4) was carried out. This stage was associated with the selection of a suitable packing media on the basis of cost involvement, performance and hydraulic efficiency. Further, to carry out this study, three different supporting media (baked clay, corrugated pipe and aqwise carrier media) having different physical properties (specific surface area and porosity) were used in the anaerobic filter. The lab-scale study revealed that the two-stage system performed better than the CST and the modified system. The two-stage system with baked clay as the supporting media in the anaerobic filter was able to fulfill the effluent disposal standards during the steady state at 24-h HRT in the summer. However, during the low-temperature conditions in the winter, a comparatively low treatment efficiency was observed. The two-stage system showed a good endurance against variable hydraulic shock loading conditions of PFF 1, 2, 4 and 6. The second-order kinetic model was found to be the most appropriate model for the two-stage anaerobic reactor. When modified Stover-Kincannon model was applied to the anaerobic-filter section of the reactor, the maximum removal rate constant (Umax) and saturation value Integrated Settler and Anaerobic Filter Based Onsite Sewage Treatment Systems Department of Civil Engineering, IIT Roorkee v constant (KB) were found to be 2.11 and 2.41 g L per day, respectively. The treatment efficiency of the system was observed to be slightly higher while using Aqwise carrier media than the baked clay media. Therefore, the media with high specific surface area (Aqwise carrier media) was selected to implement the two-stage system into the field, while baked clay media was categorized as the supporting media to develop a low-cost on-site domestic wastewater treatment system, particularly for the economically backward rural areas of the developing countries. The phase-3 was associated with performance evaluation of the two-stage system at the field scale to identify its feasibility as a package plant for the on-site treatment of wastewater generated due to household activities. Further, the performance of the two-stage system was evaluated for two different types of wastewater separately because the quality of influent wastewater also affects the performance of the system. Therefore, two differently configured anaerobic on-site package systems were developed and their performances were evaluated under actual-field conditions for the treatment of combined (black+greywater) domestic wastewater and blackwater, respectively. The first system consisting of the CST followed by an anaerobic filter (package system) was studied for the treatment of combined domestic wastewater generated by a single household. The second one consisting of the modified septic tank followed by an anaerobic filter (advanced package system) was evaluated for the treatment of blackwater generated at a residential school. On the basis of the results, the average per-capita domestic wastewater generation was found to be 140 L/person/day with the maximum contribution by the kitchen (44%). The package system achieved the average removal efficiency of 70.9±11.8, 68.7±8.5, 78.1±4.7, 23.6±9.2, 13.8±3.7 and 97.8±4.6 % for COD, BOD, TSS, TN, TP and FC, respectively. The log removals of indicator microorganisms and pathogenic parasites were 1.30±0.14, 1.10±0.11, 0.96±0.16, 1.13±0.08, 0.35±0.02, and 0.61±0.14 for TC, FC, FS, E.coli, Salmonella, and Shigella, respectively. During a long-term operation, the CST-anaerobic filter package system showed a lower pollutant removal efficiency than the lab-scale modified septic tank-anaerobic filter (two-stage) system. The lesser removal efficiency of the septic tank-anaerobic filter package system could be attributed to the fewer amount of contact between the influent wastewater and the active biomass, even during the hydraulic shock loadings due to certain household activities. Integrated Settler and Anaerobic Filter Based Onsite Sewage Treatment Systems Department of Civil Engineering, IIT Roorkee vi The advanced package system showed a good potential in removing the major pollutants, including pathogens, during long-term performance. The present system achieved comparatively higher removal efficiency of 72.6, 90.2, 81.4 and 83.2% for CODtot, CODss, BOD and TSS, respectively, as compared to the CST. In the case of the CST, the removal efficiency for all these wastewater treatment parameters varied between 30-50%. The fecal coliform removal in the advanced package system was observed to be 2.84 log. The average characteristics of the septage from the package plant for the treatment of the domestic wastewater can be categorized as low-strength fecal sludge, while the blackwater septage fell in the category of high-strength sludge. The disposal of such polluted septage in the open without any treatment may result in a great risk to the public health and surrounding environment. The advanced package system, with a simple design, easy operability, lesser maintenance, low cost, and electricity-free energy functioning, can be a suitable alternative to the CST for the treatment of high-strength domestic wastewater in the unsewered rural and peri-urban areas of the developing countries like India. Despite delivering a high removal efficiency, the advanced package system was not able to fulfill the effluent disposal standards and required post-treatment of its effluent. Therefore, in the areas where electricity supply is not an impediment, post-treatment measures can be applied to the effluent of the present anaerobic system to further enhance the treatment efficiency. Therefore, the phase-4 was associated with the investigation of applying post-treatment measures to achieve the required effluent quality to fulfill the effluent disposal standards in the areas where electricity supply is not a problem. The post-treatment study was performed in two steps using two different electricity-operated treatment systems, one for each step. In first step, contact aeration-settling tank unit was used for the treatment of the effluent from lab-scale two-stage system. In second step, electro-chemical treatment of the anaerobically treated wastewater at the field scale. The post-treatment study revealed that the contact aeration-settling tank unit (aerobic system) can be a suitable post-treatment option for the lab-scale two-stage system (anaerobic system). The combination of anaerobic and aerobic systems was capable of removing 95.3% of turbidity, 91.5% of TSS, 89% of BOD and 85% of COD while the anaerobic system operated at 24-h and the aerobic system at 2.5-h HRT. The final effluent Integrated Settler and Anaerobic Filter Based Onsite Sewage Treatment Systems Department of Civil Engineering, IIT Roorkee vii from the reactor fulfilled the effluent disposal standards, excluding phosphorous and fecal coliform. The electrocoagulation (EC) process performed satisfactorily for the removal of the organic matter (COD and BOD). It also provided the highest removal of pathogens (FC). The EC process was also found to be efficient in the removal of ammonia and phosphorous. The highest removal of pollutants was achieved at a voltage of 10V in comparison to the other applied voltages of 6 and 8.5V. The concentrations of the pollutants in final effluent after the electrochemical treatment fulfilled the effluent disposal standards prescribed in India, which state that the treated sewage should not contain BOD and TSS more than 30 and 50 mg/L, respectively. Final epilogue - The conflicts of results between the lab-scale and field-scale studies of the developed system in the actual field conditions were mainly due to the basic differences in the design of the lab-scale system and the field-scale package system. Although, the advanced package system had a media of high specific surface area and showed high pollutant removal efficiency during the laboratory-scale study, it was not able to maintain the same performance in the actual-field conditions. This was attributed to the lower depth of the media in the commercially available system than the lab-scale system. Based on the results of the actual on-site performance, the present two-stage advanced package system, with the simple and unique configuration combining the modified septic tank and an anaerobic filter, easy operability, and zero-electricity functioning, has considerable potential to be a suitable alternative to the CST for the treatment of high-strength domestic wastewater in the unsewered rural and peri-urban areas of the developing countries like India.