الفهرس 
THEORITICAL BACKGROUND AND LITRATURE REVIEWOnly 14 pages are availabe for public view
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Abstract
Management of water and wastewater tend to trigger the growing human and environmental health problems throughout the world. The problem of sanitation in Egypt increases due to the increase of population density and water consumption. Egypt, as a developing country, has around 60% of its population with access to sanitation facilities, while almost 100% of the population have access to improved water source. Accordingly, there is a need to provide sanitation facilities to the remaining 40% of the population. On-site low-cost options or decentralized sanitation systems are all become interesting solution for application and testing.
This study aims to the development and testing of a pilot scale passively aerated biological filter as a decentralized low energy system for the treatment of municipal wastewater that can be applied in small communities and/or rural areas. The pilot-plant system was designed, manufactured and installed in a nearby wastewater treatment plant at Giza Governorate. The system is capable of treating up to 5 m3/day of wastewater. It consists of inclined plate settler, followed by passively aerated biological filter (PABF) packed with supporting media (either non-woven polyester fabric (NWPF) or autoclaved aerated concrete solid waste(AACW)).
The PABF reactor has a total height of 250 cm and it consists of 4 cubic compartments ended with a cone shape settler. A spray nozzle was fixed at the top surface of the PABF for even distribution of the municipal wastewater. Each compartment in the PABF has a length of 60 cm, a width of 60 cm and 50 cm height. The total volume of each compartment is 0.18 m3 and the total volume of the PABF is 0.72 m3. Each compartment was perforated from the bottom to allow the flow of wastewater from one compartment to another. Lateral ventilation openings were constructed along the four sides of each compartment to increase natural aeration and dissolved air concentration. Each opening hole has a diameter of 2.5 cm. Also, there was a 5 cm height spacing between every two compartments in order to collect the wastewater samples and to increase the ventilation. Four sampling points were established along the axis of the reactor to collect treated wastewater at the end of each compartment. The pilot-plant was continuously fed with pre-treated real wastewater for a duration of three years.
Three case studies have been investigated to determine the optimum operating conditions for the proposed treatment system. This was carried out through changing the hydraulic retention time (HRT), organic loading rate (OLR) and packing materials in the PABF.
Case study 1:
In this case study, different HLR, HRT, and OLR were applied to the PABF and with a total media depth of 80 cm distributed equality between the 4 compartments. Four experimental runs (RUN I, II, III and IV) were extensively studied. Each run had different operating conditions in term of flow rate, HRT, and OLR. The packing media NWPF was fixed at a depth of 20 cm in each compartment. In experimental run I, the HRT was 3.5h and the OLR was 1.086 Kg BOD/m3. d. and at a flow rate of 2 m3/d. The PABF rector achieved an overall average removal efficiency of 91% for COD, 93% for BOD and 97% for TSS. The corresponding residual values were 30 mg/L, 12.3 mg/Land 5.6 mg/L. These results showed that the increase in DO concentration (which reached 6.5 mg/L) due to passive aeration was very satisfactory for biological degradation of organic matters and the removal of ammonia through nitrification process. The average ammonia concentration in the influent was 17.3 mg/L and it was reduced by 87% at the final effluent with corresponding average residual value of 2.2 mg /L.
The second experimental run was operated at a flow rate of 3 m3/d, HRT 2.3h and OLR 1.77 Kg BOD/m3. d. Results indicated that the decrease of HRT from 3.5h to 2.3h affected the performance of the PABF as the removal efficiency was decreased to 88% and 91% for COD and BOD. Their corresponding residual values were 37 mg/L and 15 mg/L. TSS removals rate was not greatly affected by the reduction of HRT. Its removal efficiency was 96% with a corresponding residual concentration of 5.8 mg/L. Although the reduction of HRT, the overall removal rate of TKN and ammonia increased in the final effluent. The removal efficiency reached 70% for TKN and 84% for ammonia compared to 66% and 87% in the first run.
In experimental run III, although the HRT was decreased to 1.72h and OLR increased to 2.15 Kg BOD/m3.d, the performance of the PABF was stable and didn’t affect with the decrease in HRT. The removal efficiencies were 87%, 91% and 96%for COD, BOD and TSS. The corresponding residual values were 42 mg/L, 18.3 mg/L and 4.7 mg/L. The overall removal rate of TKN and ammonia was significantly constant and didn’t affected by the reduction of the HRT. The average removal efficiency of TKN was 70% and analysis of ammonium nitrogen removal showed that NWPF provided favorable conditions for the nitrifying bacteria, as the mean ammonium nitrogen removal rate for this material was 85%.
In the experimental run VI, the HRT decreased to 1.38h and OLR increased to 2.9 Kg BOD/m3. d as a result of increasing flow rate to 5 m3/d. The results indicated that the reduction of HRT had a negative effect on the performance of the PABF. The removal efficiencies of COD, BOD and TSS decreased to 84%, 88% and 88%.
Corresponding residual values were 49 mg/L, 20 mg/Land 20.7 mg/L. Low removal efficiency of organics and suspended solids at a high OLR of 2.9kgCOD/m3 d. resulted from excess biomass accumulation, filling the pores of the NWPF packing material and reducing the mass transfer capabilities. Also, the removal efficiency decreased to 64% for TKN and 75% for ammonia. Their corresponding residual values were 9.9 and 5.1 mg/L, respectively. Nitrification efficiency was decreased to about 70%. These results indicated that the removal of ammonia through nitrification process was affected by reduction of HRT. In conclusion, the results indicated that Run III showed a perfect nitrification process although the short HRT and high loading rate.
Case study 2:
In this case study, different media depths of NWPF were studied. The volume of the NWPF was changed from 20 to 30 cm in each compartment with a total media depth of 120 cm all over the reactor. The PABF was operated at the pre-determined optimum operating conditions namely; HLR (11.11 m3 /m2. d), HRT (2.3h) and OLR (2.21Kg BOD/m3. d). During this case study, the PABF achieved a high performance for organic matter removal where the COD of the final effluent remained below 20 mg /L. Results indicated that the PABF achieved an overall average removal efficiency of 95% for COD, 95% for BOD and 97% for TSS. The corresponding residual values were 16.45 mg/L, 7.5 mg/Land 3.1 mg/L. The increase in the removal of organic matter in terms of COD and BOD occurred as a result of increasing the media depth by 50% (compared to 80 cm media depth) and consequently increasing the NWPF surface area. Also, increasing the NWPF depth in all compartments achieved high removal rate of TKN and ammonia nitrogen. The average removal efficiency of TKN was 87 % with a corresponding residual value of 4.48 mg/L. The ammonia nitrogen removal increased with increasing the NWPF depth. This case study provided favorable conditions for the nitrifying bacteria, as the mean ammonium nitrogen removal rate reached 96%.
Case study 3:
In this case study autoclaved aerated concrete solid waste
(AACW) was used as a bio-carrier at 30 cm media depth in each compartment with an overall media depth of 120 cm. PABF was operated at the pre-determined optimum operating conditions in case study I& II. The HLR was 11.11 m3 /m2. d, HRT 2.3h and OLR 2.25Kg BOD/m3. d. Physical characteristics (BET, XRD, EDX and SEM) indicated that AACW has a rough surface with a total surface area around 42.8 m2/g, a low bulk and apparent density. These characteristics are propitious to develop many microbial communities and to improve the capacity of biofilm layers for pollutant’s removal efficiencies.
Results indicated that the use of AACW in PABF reactor achieved a final effluent with an overall average percentage removal values of COD tot, COD sol, BOD, and TSS of 90.48%, 89.8 %, 90.04 % and 92 %. Their corresponding residual values were 34.5 mg/L, 17.6 mg/L, 19.5 mg/L and 11.2 mg/L. The rough surface of AACW and its internal pores structure are uniform and well developed so that soluble organic matter, nutrient substances and tiny suspended particles can reach the deep holes at the surface which resulted in the enhancement of mass transfer efficiencies. Results indicated also that the use of AACW bio-carrier resulted in 80% and 89% reductions of TKN and NH4+-N with average residual values of 7.1and 3.3 mg N/L.
Modeling and simulation of PABF
PABF was successfully modelled using an attached growth unit process (a modified trickling filter) in GPS-X. At steady state, effluent results from field-scale systems were accurately modelled as were carbon and nitrogen (390mg/l and 35 mg/l). It was challenging to get the passive aeration process working in GPS-X. However, once the PABF model was built, it was quick and easy to calibrate and validate the model. The model was initially calibrated from experimental data, where real municipal wastewater was treated by the PABF unit. The average measured removal efficiency for COD was 91.8% compared with the modelled removal efficiency 90.9%. The removal efficiency for TKN was 71% and the modelled 98% for modelled value. Similarly, good agreement between measured and predicted ammonia removal values was observed. (86.6% and 88% for measured and predicted removal efficiency). Six different simulation scenarios were evaluated to investigate the performance of PABF among various operating parameters. The scenarios were based on increasing the influent flow rate by 15%, 30%, 40%, 50%, 60%, and 75% and consequently increasing the HLR and OLR within the same pattern. It was possible to estimate the optimal operating conditions which can achieve an effluent quality required for different purposes.
5.2 Conclusion
1. Passively aerated filter (PABF) proved to be a very promising and applicable technology for decentralized municipal wastewater treatment.
2. The system saved the energy which is required for any conventional aerated biological reactors. There is no need for aeration in PABF as the oxygen is naturally diffused from ambient air into the reactor creating ideal conditions for bacteria to oxidize the organics and ammonia.
3. The media depth, HRT, and D.O concentrations showed a linear correlation with the PABF performance for biodegradation of the organic and nitrification process.
4. PABF design configuration with lateral opening increased the oxygen supply to the microorganisms colonized in the bio-bed and accordingly enhanced its activity for biodegradation and nitrification.
5. The use of NWPF, as a novel bio-bed, proved to be a very efficient material for the growth of microorganisms and consequently more removal of pollutants due to its high surface area, porosity and rough surface which enhance the adhesion of microbial biomass.
6. The use of NWPF with high specific surface area (2000 m2/m3) leads to a strong adhesion of microbial biomass onto the NWPF, promoting the effective physical entrapment/ adsorption and subsequent bonding (chemical/ electrostatic and Vander Waals forces) between biomass and media which lead to immobilization of attached biomass.
7. This study proved the success of the utilization of autoclaved aerated concrete solid waste (AACW) as a low-cost bio-carrier in passively aerated biological filter (PABF) treating wastewater.
8. The extensive characterization and performance data of AACW as a bio carrier in biological filters will offer a large opportunity in future for the utilization of AACW solid waste as a bio- carrier in biological reactors.
9. PABF reactor packed with AACW also proved to be a promising and reliable technology for municipal wastewater treatment where water and energy are scarce.
10. The integration of AACW in PABF enhanced the performance of the bio- reactor and minimize the solid wastes coming from construction and demolition.
11. Results indicated that the carrier properties has a considerable effect on COD and TN removal efficiencies in the bio-filters. They determine the mass transfer of wastewater constituents, the permeable capacity of biofilm and the extent of contact reactions.
12. Good removal efficiency of COD, BOD5, NH4-N, and TSS were achieved, they reached 91%, 93%, 87% and 97%, respectively.
13. PABF profile results showed that the major part of COD was removed in the upper portion of the system as most of the organic matter represented by VSS was trapped in the first and second compartments, while the nitrification process took place in the lower part of PABF.
14. The process of nitrification is also confirmed by the decrease in the ammonia content and increase of nitrate concentrations in the treated effluent. The key argument being that the efficient oxygen transfer afforded by the media design is sufficient to satisfy heterotrophs and autotrophs oxygen demand simultaneously.
15. The excess sludge produced from PABF was 315, 325 and 380 ml/d at OLR 1.77, 2.15 and 2.9 Kg BOD/m3.d which is very low compared to conventional treatment systems.
16. A model of PABF was successfully calibrated using attached growth process object in GPS-X and the model successfully predicted effluent characteristics for carbon and nitrogen.
17. Good calibration was achieved based on influent data. With this in mind, it appears that for the calibration of the PABF, extremely detailed influent/effluent breakdown data is not critical.
18. The model was used to simulate various scenarios for optimizing the operation of the field-scale system indicating that Such models, while not providing in-depth analysis of biofilm characteristics, can be used to enhance reactor operation and inform future studies without the need to develop a bespoke model.
Finally, the effluent quality from the treatment system complies with National Regulatory Standards for either discharge into surface water and/or reuse of treated effluent in irrigation after disinfection. Application of such treatment system is governed by many factors such as legal status, land availability, ease of operation maintenance capital and operation cost.