Performance characteristics of aerobic/anoxic sludge digestion at elevated temperatures.

Laboratory experiments were conducted to study the performance of aerobic/anoxic digestion of mixed primary and waste activated sludge compared to aerobic digestion alone in terms of solids destruction, organic reduction, nutrient removal, filtrate quality and sludge dewaterability. The process performance was examined at different solids residence times (10 and 20 d), temperatures (20, 30 and 40 degrees C) and anoxic cycle length (0, 8, 12 and 16 h). Both modes of operation gave comparable solids destruction results. The volatile suspended solids (VSS) reduction was affected by temperature with the highest VSS reduction reported at 30 degrees C (42.4%), while solid retention time (SRT) did not seem to affect the process as indicated by the percent VSS removal. Biodegradable solids destruction followed a first-order kinetic model. Aerobic/anoxic sludge digestion is advantageous to mere aerobic digestion since it improves sludge dewaterability and filtrate quality. Alternating aerobic/anoxic operation can conserve most of the influent alkalinity and maintain near neutral pH conditions over prolonged periods. Optimum process performance was observed at 20 degrees C with SRT of 10 days and anoxic cycle length of 8 hours.

Coagulant selection and sludge conditioning in a slaughterhouse wastewater treatment plant.

Attempts were made in this study to examine the effectiveness of polymer addition to the aeration tank effluent prior to sludge flotation as practiced in a slaughterhouse wastewater treatment plant. The plant currently uses 10 mg/l of polymer prior to sludge flotation, but alternative, less-expensive, chemicals such as alum could be equally effective. Therefore, experiments were conducted using the Standard Jar test to determine the performance of both alum (Al2SO4.6H2O) and organic polymer. The dosages used for alum ranged between 0 and 1000 mg/l, whereas polymer dosages varied between 0 and 90 mg/l. The (optimal) removal efficiency for suspended solids in the mixed liquor was obtained at 400 mg/l for alum and 30 mg/l for polymer. It is evident that addition of alum or polymer results in significant removal of suspended solids reaching up to 99% for alum and 96% for polymer but alum produced a more compacted sludge. Removal of filterable COD was much lower in both cases since the chemicals used target the colloidal and suspended portion of the COD rather than the soluble (filterable) part of the COD.

Application of granular media filtration in wastewater reclamation and reuse.

Laboratory experiments were conducted at room temperature (20-25 degrees C) using four identical filter columns made of Plexiglas, each of 1 m height and 15 cm internal diameter, packed with granular media of 70 cm depth. Each filter was operated at a constant filtration rate, thus four rates were tested in the range of 2-15 m(3) m(-2) d(-1). Mono-media (sand) and dual-media (sand and anthracite) were tested and three types of municipal wastewaters, namely raw, primary and secondary-treated effluents were applied. The results obtained indicate that considerable improvements in effluent quality could be attained by tertiary sand filtration. Removal of solids, organics and bacteria was not significantly affected by the increase in filtration rate from 2 to 15 m(3) m(-2) d(-1). The highest removal efficiency was obtained at low filtration rate of 2 m(3) m(-2) d(-1), but higher filtration rates achieved acceptable removal efficiencies and provided effluents of good quality to satisfy the irrigation water quality standards. Since the conventional sand filters in wastewater treatment plants operate at a rate in the range of 2-5 m(3) m(-2) d(-1), utilization of high rate filtration is advantageous and would result in significant cost savings. However, with high filtration rates the filters require more frequent backwashing. Dual-media filters achieved 50% reductions in BOD suggesting that filtration could be used to treat primary effluents in emergency cases.

Heavy metals content of municipal wastewater and sludges in Kuwait.

Municipal wastewater may contain heavy metals, which are hazardous to the environment and humans. With stringent regulations concerning water reuse and sludge utilization in agriculture, there is a great need to determine levels of heavy metals in liquid wastes, sludges and agricultural crops. The state of Kuwait has programs to utilize waste sludge produced at wastewater treatment plants as soil conditioner and fertilizer for greenery and agricultural development projects and to reuse treated wastewater effluents in irrigation. The common metals found in Kuwait's raw wastewater and sludge are Cd, Cr, Cu, Hg, Ni, Pb, and Zn. The effects of accumulation of heavy metals in soil are long lasting and even permanent. In this study, the variations in the concentration levels of heavy metals were measured in wastewater and sludge produced at Ardiya municipal wastewater treatment plant in Kuwait. A relationship was observed between the concentrations of heavy metals in treated wastewater and sludge used for agriculture and the level of accumulated heavy metals found in residual tissues of some crops.

Enhancement of carbon and nitrogen transformations during composting of municipal solid waste.

Appropriate management of municipal solid waste (MSW) is viewed as an integrated system encompassing waste minimization, recycling, and resource recovery. This has increased the interest in the composting process, in which the biodegradable portion of MSW is biologically converted into a valuable product that can be utilized as a soil conditioner or fertilizer. Effective operation of the composting process can be achieved by determining the optimal conditions for the controlling parameters in order to enhance the biological reactions during composting. Information on biodegradation rates are also essential for process design. Therefore, this study was conducted in order to examine the factors affecting process performance and transformation of essential elements of MSW such as carbon and nitrogen. A pilot study of in-vessel aerobic composting of MSW was carried out and the operating parameters, which affect biodegradation of carbonaceous and nitrogenous matter were studied using a specially designed rotating drum and a carbon-rich MSW. The effects of temperature, moisture content, and air flow rate were examined. The performance of the composting process was evaluated by measuring the carbon and nitrogen content during experiments, each of which lasted for 30 days. The study showed that enhanced transformations of carbon and nitrogen occur at 55 degrees C for temperature, 60% for moisture content, and 10L/kgh for air flow rate using the food-rich MSW generated in the Gulf countries.

Ion exchange extraction of heavy metals from wastewater sludges.

Heavy metals are common contaminants of some industrial wastewater. They find their way to municipal wastewaters due to industrial discharges into the sewerage system or through household chemicals. The most common heavy metals found in wastewaters are lead, copper, nickel, cadmium, zinc, mercury, arsenic, and chromium. Such metals are toxic and pose serious threats to the environment and public health. In recent years, the ion exchange process has been increasingly used for the removal of heavy metals or the recovery of precious metals. It is a versatile separation process with the potential for broad applications in the water and wastewater treatment field. This article summarizes the results obtained from a laboratory study on the removal of heavy metals from municipal wastewater sludges obtained from Ardhiya plant in Kuwait. Data on heavy metal content of the wastewater and sludge samples collected from the plant are presented. The results obtained from laboratory experiments using a commercially available ion exchange resin to remove heavy metals from sludge were discussed. A technique was developed to solubilize such heavy metals from the sludge for subsequent treatment by the ion exchange process. The results showed high efficiency of extraction, almost 99.9%, of heavy metals in the concentration range bound in wastewater effluents and sludges. Selective removal of heavy metals from a contaminated wastewater/sludge combines the benefits of being economically prudent and providing the possibility of reuse/recycle of the treated wastewater effluents and sludges.

Evaluation of landfill leachate in arid climate-a case study.

Generation of leachate from municipal solid waste (MSW) landfill in arid regions has long been neglected on the assumption that minimal leachate could be formed in the absence of precipitation. Therefore, a case study was conducted at two unlined MSW landfills, of different ages, in the state of Kuwait in order to determine the chemical characteristics of leachate and examine the mechanism of leachate formation. Leachate quality data were collected from both active and old (closed) landfills where co-disposal of MSW and other solid and liquid wastes is practiced. The analysis of data confirms that leachates from both landfills are severely contaminated with organics, salts and heavy metals. However, the organic strength of the leachate collected from the old landfill was reduced due to waste decomposition and continuous gas flaring. A significant degree of variability was encountered and factors which may influence leachate quality were identified and discussed. A water balance at the landfill site was assessed and a conceptual model was presented which accounts for leachate generation due to rising water table, capillary water and moisture content of the waste.

Removal of organics from wastewater using a novel biological hybrid system.

This paper summarizes the results obtained using the hybrid aerated submerged fixed-film (HASFF) process. HASFF is an innovative system comprising a four-compartment reactor having an array of fixed ceramic plates maintained under diffused aeration to support attached biomass, with activated sludge recycle to promote suspended growth in the reactor. Wastewater from a municipal treatment plant was fed to the reactor and its activated sludge was used for recycling in the hybrid system. Four pilot units were operated in the plant at various hydraulic retention times, HRTs (2, 4, 6 and 8 h), using primary-settled wastewater under organic loading rates up to 0.7 g BOD/gMLTVS. d. Data obtained showed that the overall BOD percentage removal efficiencies were consistently above 94.0% at all HRTs including the 2 hours while the COD percentage removal efficiencies ranged between 65.7-76%. The effluent's mean filtered BOD concentration ranged between 4.5-7.5 mg/l whereas the mean filtered COD concentration ranged between 70.0-89.6 mg/l. Increasing the hydraulic loading rate by four-folds from 0.08 to 0.32 m3/m2.d had a minor effect on the unit's BOD and COD percentage removal efficiencies indicating a robust biological process that is resilient to hydraulic shock loads, thereby offering a viable upgrading option.

Biomass retention and performance of anaerobic fixed-film reactors treating acetic acid wastewater.

An acetic-acid-based synthetic wastewater of different organic concentrations was successfully treated at 35 degrees C in anaerobic downflow fixed-film reactors operated at high organic loading rates and short hydraulic retention times (HRTs). Substrate removal and methane production rates close to theoretical values of complete volumetric chemical oxygen demand (COD) removal and maximum methane conversion were obtained. A high concentration of biofilm biomass was retained in the reactor. Steady-state biofilm concentration increased with increased organic loading rate and decreased HRTs, reaching a maximum of 8.3 kg VFS/m(3) at a loading rate of 17 kg COD/m(3) day. Biofilm substrate utilization rates of up to 1.6 kg COD/kg VFS day were achieved. Soluble COD utilization rates at various COD concentrations can be described by half-order reaction kinetics.