Paris Melidis

Surplus activated sludge dewatering in pilot-scale sludge drying reed beds

A pilot-scale experiment on dewatering of surplus activated sludge (SAS) is presented, where two pilot-scale vertical flow, sludge drying reed beds (SDRBs), planted with Phragmites australis are used. The bottom of the beds is filled with cobbles, connected to the atmosphere through perforated PVC ventilation tubes, in order to achieve oxygen diffusion through the overlying porous medium that is colonized by roots and an abundant nitrifying biomass. Two layers of gravel, of decreasing size from bottom to top, make the drainage layer where the reeds are planted. The two beds were fed according to the following cycle: one week feeding with SAS at rates one 30 kg/m(2)/year and the other 75 kg/m(2)/year, and resting for three weeks. The results show that planted SDRBs can effectively dewater SAS from domestic sewage, the produced residual sludge presents a high dry weight content, the degree of volume reduction depends upon the initial SAS concentration and can be of the order of 90%, and decomposition of organic matter and high levels of mineralization can be achieved. Furthermore, the percolating water is not septic. The fertilizer value of the treated SAS, which contains no added chemicals, is comparable to that of SAS treated by other methods.

Development and implementation of microbial sensors for efficient process control in wastewater treatment plants

This paper demonstrates the functionality, laboratory testing and field application of a microbial sensor, which can be modified to monitor organic pollution extent, toxicity and over-(under)load of wastewaters both under anaerobic and aerobic conditions. Since nitrification is related to protons formation and the addition of alkaline is necessary for pH control, an aerobic biosensor monitoring Na2CO3 consumption was developed and practically implemented to control the nitrification process. As CO2 is the respiration product from aerobic degradation which can be correlated to the organic pollution extent, the previous biosensor was modified to monitor and measure the online toxicity and BOD/COD. Under anaerobic conditions, the online measurement of NaOH consumption and biogas production allowed the detection of toxicity incidents and over-(under)load in the influent. Such toximeters get in contact with the wastewater the earliest possible, providing sufficient time for protection of sensitive biological wastewater treatment processes and for the implementation of control and management strategies.

Sewage pre-concentration for maximum recovery and reuse at decentralized level

Pre-concentration of municipal wastewater by chemically enhanced primary treatment (CEPT) was studied under controlled laboratory conditions. Both iron and aluminium-based coagulants were examined at gradually increasing concentrations (0.23, 0.35, 0.70 and 1.05 mmol/L). The CEPT sludge generated from different coagulation experiments was digested in batch anaerobic reactors, while the supernatant was tested in a dead-end microfiltration setup. The results of the study show that biogas yield was dramatically decreased (from 0.40 to 0.10 m(3)/kg chemical oxygen demand of influent) with increasing coagulant dose. In contrast, supernatant filterability was improved. Based on the laboratory results, a conceptual design was produced for a community of 2000 inhabitant equivalents (IE), using CEPT technology (at low coagulant dose) with anaerobic digestion of the concentrates. According to this, the capital and operational costs were 0.11 and 0.09 €/m(3), respectively. The biogas generated is used for digester heating and the overall process is energy self-sufficient. At a small-scale and in private applications, CEPT technology is preferably operated at higher coagulant dose, followed by membrane filtration for water reuse. Accordingly, sewage purification and reuse is possible without implementing aerobic biological processes.

Process control, energy recovery and cost savings in acetic acid wastewater treatment

An anaerobic fixed bed loop (AFBL) reactor was applied for treatment of acetic acid (HAc) wastewater. Two pH process control concepts were investigated; auxostatic and chemostatic control. In the auxostatic pH control, feed pump is interrupted when pH falls below a certain pH value in the bioreactor, which results in reactor operation at maximum load. Chemostatic control assures alkaline conditions by setting a certain pH value in the influent, preventing initial reactor acidification. The AFBL reactor treated HAc wastewater at low hydraulic residence time (HRT) (10-12 h), performed at high space time loads (40-45 kg COD/m(3) d) and high space time yield (30-35 kg COD/m(3) d) to achieve high COD (Chemical Oxygen Demand) removal (80%). Material and cost savings were accomplished by utilizing the microbial potential for wastewater neutralization during anaerobic treatment along with application of favourable pH-auxostatic control. NaOH requirement for neutralization was reduced by 75% and HRT was increased up to 20 h. Energy was recovered by applying costless CO(2) contained in the biogas for neutralization of alkaline wastewater. Biogas was enriched in methane by 4 times. This actually brings in more energy profits, since biogas extra heating for CO(2) content during biogas combustion is minimized and usage of other acidifying agents is omitted.

Microbial Sensors for Monitoring and Control of Aerobic, Anoxic, and Anaerobic Bioreactors in Wastewater Treatment

This article reviews development and implementation of microbial sensors, which monitor organics extent, toxicity, and over-(under)load of wastewaters, aiming to control the aerobic, anoxic, and anaerobic biological processes in wastewater treatment. Emphasis is placed on an alternative biosensor that performs process control in wastewater treatment and can be modified to operate under aerobic or anaerobic conditions. Its working principle is based on the online measurement of respiratory product from aerobic biodegradation (CO 2 ), produced by microbial respiration activities, as well as on the online measurement of alkaline consumption for pH control. As nitrification is related to proton formation and alkaline addition is necessary for pH control, an aerobic biosensor monitoring Na 2 CO 3 consumption was developed and implemented for nitrification control. CO 2 can also be correlated to the organics extent so that the biosensor can monitor and measure online toxicity and biochemical oxygen demand/chemical oxygen demand. Under anaerobic conditions, the online measurement of NaOH consumption and biogas production allows the detection of toxicity incidents and over-(under)load in the influent. Finally, CH 4 concentration can be monitored on-line in the off-gas of an anaerobic denitrification reactor to optimize addition of external electron donor for denitrification. Such biosensors provide sufficient time for protection of sensitive biological wastewater-treatment processes and for implementation of control and management strategies.