PhD by Sunil Prasad Lohani, Nepal (PPRE 2006/08) at KU, Nepal with the support from University College of Southeast Norway, Norway and Karlsruhe Institute of Technology, Germany (as DAAD sandwich PhD research scholar)
This research presents results and discussion to understand the design and operational parameters for Anaerobic Digestion (AD) of domestic wastewater at ambient temperature conditions of an average about 10 oC to 22 oC for winter and summer, respectively. The main focus of this study was on the key operational parameters (hydraulic retention time–HRT, influent Chemical Oxygen Demand–CODin, Organic Loading Rate–OLR and Sludge retention Time–SRT) and design parameters (reactor height, start-up history and flow mode of operation- continuous feed and pulse feed mode) while applying Up-flow Anaerobic Sludge Blanket (UASB) reactor fed with Septic Tank (ST) treated domestic wastewater.
The experimental investigation was performed using existing ST with active volume of about 13.5 m3 and 3 pilot-scale UASB reactors. Two cylindrical UASB reactors of 250 L (height 1 m and diameter 0.56 m made up of PVC barrel) including 50 L headspace and one cylindrical UASB reactor of 550 L (height 1.7 m and diameter 0.64 m made up of mild steel) including 80 L headspace were used. One of the 250 L reactors was fed continuously whereas the others were fed intermittently using an automatic timer for pumping the wastewater at specific intervals. The UASB reactors continuous feed 250 L (R1), pulse feed 250 L (R2) and pulse feed 550 L (R3) were inoculated with, respectively 20% of the active working volume of the reactor for R1 and R2 and 15% of the active working volume of the reactor for R3 by an anaerobic slurry obtained from manure biogas plant. The reactor (R1) operated with HRT dropping from 4d to 4 h HRT for about 6 months. The reactor (R2) operated with HRT dropping from 10 d to 4 h HRT for about one and half years of operation. The reactor (R3) was operated at 4 d and 1 d HRT for about 6 months.
The modelling and simulation were performed using ADM1 model to investigate whether ADM1 model can be used as useful tool to evaluate and design of low cost UASB and ST-UASB systems for fluctuating climatic environment. The ADM1 was implemented in the software AQUASIM 2.1.The model was used to simulate various loads at 12, 8 and 6 h HRT with varying SRT and temperatures.
The performance of continuous and pulse feed mode of operation monitored in the first six months of operation in a 250 L UASB reactor shows that pulse feed mode of operation is better. Moreover, ADM1 modelling too shows that pulse feed mode of operation is suitable for efficient conversion of COD to biogas. Among the different pulse intervals and lengths simulated, biogas production at 2 h pulse interval and 10 minutes pulse length was the highest and was about 20% higher than the continuous operation, whereas COD removal was about 8% less.
The influence of start-up history and reactor height monitored in 250 L and 550 L reactors at 4 and 1 d HRT shows that start-up history and reactor height had no significant influence on the process performance. The removal efficiencies of Total Suspended Solid (TSS) and Total Chemical Oxygen Demand (CODT) were about 59–68% and 54–59%, respectively, for both reactors at both (4 and 1 d) HRT. The average biogas yields were almost the same at both HRT, representing 31(±3) % of influent CODT.
The load limit test of 250 L pulse feed UASB reactor under 18 to 4 h HRT and 1 to 4 kgCOD/(m3.d) Organic Loading Rate (OLR) reveals that HRT had no significant influence on removal efficiencies in the UASB at HRT ≥ 6h and the process established reasonably stable UASB treatment at HRT= 6 h, indicating a hydraulic load design limit.
The ADM1 applied to the UASB reactor treating domestic wastewater pretreated by a ST was evaluated based on data from the 250 L pulse feed reactor (R2). The wastewater influent degradable organics characteristics and Sludge Retention Time (SRT) were established by several trial and errors simulation to establish appropriate values for parameters not measured experimentally. The model was verified against varying load experimental results to ensure its usefulness for a wide range of relevant loads. The best fit was obtained using 60 d SRT and parameters recommended for mesophilic conditions, with average experimental values within 10% of the simulated results. Furthermore, ADM1 was applied to an UASB and ST-UASB combined process treating domestic wastewater at different temperatures (10, 15 and 20 oC) and loads using temperature compensation kinetics for low temperature conditions. The UASB model applied was quite reasonable in predicting the behavior of such a process to treat domestic wastewater. The ST behavior was not equally well predicted by ADM1 as the UASB and reasons are discussed. The overall ST-UASB model underestimates COD accumulation and COD removal, while overestimating biogas production by up to 15%.
The study suggest that AD of ST treated domestic wastewater by a UASB reactor could be a suitable low cost treatment option for varying load and temperature conditions. The ADM1 modelling of UASB reactor and ST-UASB combined system at mesophilic and low temperatures conditions can become a useful tool to evaluate and design of low cost UASB and ST-UASB systems for fluctuating climatic environment such as Nepal.
Keywords: Anaerobic Digestion, septic tank, UASB, ADM1, SRT, domestic wastewater, low temperature
Sunil Prasad Lohani, PhD
Department of Mechanical Engineering, Kathmandu University