Anaerobic Sewage Treatment for Sustainable Water Reclamation in Jordan
Investigators (most current known information)
The reuse of treated wastewater is an important resource in the total water budget of Jordan. This project explores the use of an anaerobic treatment system known as the upflow anaerobic sludge bed (UASB) reactor as part of the overall sewage treatment strategy in Jordan. The advantage of anaerobic pre-treatment is to save energy and reduce excess biosolids production. Jordanian sewage has a high strength due to scarce water resources and in the highly populated plateau the sewage is colder than in neighboring middle eastern countries. Therefore the study explores adaptations of the UASB technology to improve its performance with higher strength sewage and at lower temperatures. This a collaboration between the Department of Chemical and Environmental Engineering (CHEE) at the University of Arizona (UA) and the Water and Environment Research Study Center (WERSC) of the University of Jordan (UJ). The U.S. partner has the task to evaluate the water quality achieved after aerobic post treatment of anaerobic effluents as well as evaluate innovative methodologies of nutrient nitrogen removal. The Jordanian partner has the task to evaluate several alternative designs of the anaerobic pretreatment system to improve the removal of suspended solids removal and degradation.
The U.S. Partner conducted two major research tasks. The first task explored the treatment of a synthetic high strength sewage in a laboratory-scale UASB with aerobic activated sludge (AS) post-treatment. The synthetic sewage contained a high level of protein (gelatine). The UASB-AS system operated reliably in the removal of organic constituents measured as chemical oxygen demand (COD), volatile fatty acids (VFA) and protein. The high protein sewage however, most of the organics were removed in the anaerobic stage and converted to methane; whereas organic residuals were polished in the AS stage. The AS stage was able to remove high levels of organic compounds when the UASB was overloaded. The methanogenic biofilms did not grow in the presence of high protein sewage, instead there was a slow erosion of the biofilms, such that progressively the organic loading rate had to be lowered to maintain a high COD removal efficiency. Concerning the nitrogen, protein-N was hydrolyzed and mineralized to ammonium in the anaerobic stage. The ammonia in turnwas converted to nitrite and nitrates in the aerobic stage. Partial nitrification to nitrite was the dominant process when the AS received a high organic loading or when the dissolved oxygen (DO) was purposefully maintained below 2 mg L-1. Incomplete N-balances at low DO indicated that denitrification was taking place in the AS reactor. When high levels of DO were supplied ammonia was stoichiometrically converted to nitrate.
In the second research tasks, the impact of nitrate and nitrite (collectively NOx- compounds) on methanogenesis was explored since anaerobic bioreactors can potentially support simultaneous microbial processes of denitrification and methanogenesis. The methanogenic toxicity of the NOx- compounds to anaerobic granular biofilms and municipal anaerobic digested sludge with two types of substrates, acetate and hydrogen, was studied. The inhibition was the severest when the NOx- compounds were still present in the media (exposure period). During this period, 95% or greater inhibition of methanogenesis was evident at the lowest concentrations of added NO2- tested (7.6 to 10.2 mg NO2- -N l-1) or 8.3 to 121 mg NO3- -N l-1 of added NO3- , depending on substrate and inoculum source. The inhibition imparted by NO3- was not due directly to NO3- itself, but instead due to reduced intermediates (e.g., NO2-) formed during the denitrification process. The toxicity of NOx- was found to be reversible after the exposure period. The recovery is attributed to the metabolism of the NOx- compounds. The final methane yield was inversely proportional to the amount of NOx- compounds added indicating that they were the preferred electron acceptors compared to methanogenesis.
The Jordanian partner conducted three research tasks. In the first task, an anaerobic filter (AF) was used for polishing effluent of a UASB reactor treating strong sewage at an average water temperature of 23°C. The UASB-AF system was operated with sludge discharge taking place only from the AF reactor. It was hypothesized that better removal of suspended COD (CODss) fraction from the system can be achieved by increasing sludge discharge frequency from the AF reactor. For this purpose, sludge discharge frequencies of once, twice and thrice per day from the AF reactor were investigated. Results showed that average total COD (CODtot) removal efficiency of the system increased from 62% when sludge was discharged once per day from the AF reactor up to 67% when sludge was discharged twice and thrice per day. However, the increase in CODtot removal was not due to increased removal of CODss fraction; but mainly due to development of an active biomass on the filter media, which increased the removal efficiency of soluble COD fraction. Evidence was obtained showing that the AF reactor was able to reduce pathogens. The AF reactor achieved 1-2 log reduction in fecal coliform counts.
In the second Jordanian task, a two-stage UASB-AF reactor system was operated at 23°C with concentrated domestic sewage treatment at hydraulic retention times (HRT) of 15 and 4 h, respectively. Excess sludge from the downstream AF stage was returned to the upstream UASB reactor. The aim was to obtain higher sludge retention time (SRT) in UASB reactor for better conversion of suspended COD. The UASB-AF system removed 55% and 65% of the total COD (CODtot) and suspended COD (CODss), respectively. The calculated SRT in the UASB reactor ranged from 20-35 days. The AF reactor showed the potential to remove washed out sludge from the first stage reactor with CODss removal efficiency as high as 85%. The VFA concentration in the effluent of the AF was 39 mg COD/l compared with 78 mg COD/l measured for the influent. The slightly higher CODtot removal efficiency obtained in this study compared with 50% for a single stage UASB reactor was achieved with a 17% reduction in the total reactor volume.
In the third task, the treatment of sewage in a UASB at a very cold temperature of 15°C was investigated. The UASB was integrated with a sludge digester to determine if the UASB performance could be improved. Incorporating a digester with the UASB reactor for sewage treatment at 15°C significantly improved the removal efficiency of the CODtot in the UASB reactor from 20% to 37%. However, the performance of the UASB reactor was not stable when operated at such a low temperature and sludge washout was frequently noticed.
The results taken as a whole suggest, that at moderate temperatures of 23°C or higher, UASB shows promise as a sewage pretreatment technology, enabling reductions in aeration costs, recovery of methane as an energy resource and preparation of sewage for nutrient nitrogen removal.
Articles in Journals
Jasoni, R.L., J.D. Larsen, L.F. Fenstermaker, E. Knight, J. Grünzweig, and J.A. Arnone III. 2009. Effects of increased rainfall and atmospheric N deposition on net ecosystem CO2 exchange and evapotranspiration in a Mojave Desert ecosystem.” Global Change Biology. (to be submitted).
Arnone III, J.A., R.L. Jasoni, L.F. Fenstermaker, J.D. Larsen, and J. Grünzweig. 2009. “Seasonal and diurnal variation in net CO2 fluxes of Mojave Desert soil surfaces covered with cryptobiotic crusts: differential impacts of water pulses in winter and summer.” Oecologia. (to be submitted)
Grünzweig J. et al. 2009. “Environmental and biotic controls on soil and ecosystem net CO2 fluxes in the Negev Desert.” Global Change Biology. (to be submitted)
Grünzweig, J. 2008. Seminar at the Weizmann Institute of Science. May. Rehovot, Israel.