Tobias Hey

Potential of combining mechanical and physicochemical municipal wastewater pre-treatment with direct membrane filtration

ABSTRACT At a full-scale wastewater treatment plant, raw municipal wastewater from the sand trap outlet was mechanically and physicochemically pre-treated before microfiltration (MF) in a large pilot-scale study. MF was performed using a low transmembrane pressure (0.03 bar) without backflushing for up to 159 h (∼6.6 d). Pre-filtration ensured stable MF operation compared with the direct application of raw wastewater on the membrane. The combination of physicochemical pre-treatment, such as coagulation, flocculation, and microsieving, with MF meets the European and Swedish discharge limits for small- and medium-sized wastewater treatment plants (WWTPs). The specific electricity footprint was 0.3–0.4 kWh·m−3, which is an improvement compared to the median footprint of 0.75 kWh·m−3 found in 105 traditional Swedish WWTPs with sizes of 1500–10,000 person equivalents. Furthermore, the biological treatment step can be omitted, and the risk of releasing greenhouse gases was eliminated. The investigated wastewater treatment process required less space than conventional wastewater treatment processes, and more carbon was made available for biogas production.

Determining short chain fatty acids in sewage sludge hydrolysate: a comparison of three analytical methods and investigation of sample storage effects.

In anaerobic digestion, the production of short-chain fatty acids (SCFAs) can be beneficial or harmful to the overall process, depending on the concentration of accumulated acids. Therefore, the accurate determination of the SCFA concentration in both fresh and stored sludge hydrolysates is important. To select a suitable method for monitoring SCFAs during the anaerobic digestion of sewage sludge, the accuracy of three available analytical methods, including 5 pH point acid titration (TITRA5), gas chromatography (GC), and spectrophotometry, were compared in the present study. The results revealed that TITRA5 and GC displayed better agreement in the achieved measurements and higher precision and accuracy than the spectrophotometric assay, as supported by the application of different statistical models. TITRA5 excelled in titrating unfiltered hydrolysate while simultaneously measuring the alkalinity, whereas the GC method provided detailed information on the contribution of different fatty acids to the total acidity. In contrast, the spectrophotometric assay suffered from many forms of interference, depending on the samples matrix. SCFA production followed the pattern of enzymatic reactions and fitted the Michaelis-Menten model. In addition to promoting TITRA5 as an accurate and robust analytical tool for routine SCFA analyses, this comparative study also demonstrated the possibility of storing hydrolysate samples at different temperatures and durations without altering the SCFA measurements.

Evaluation of direct membrane filtration and direct forward osmosis as concepts for compact and energy-positive municipal wastewater treatment

ABSTRACT Municipal wastewater treatment commonly involves mechanical, biological and chemical treatment steps to protect humans and the environment from adverse effects. Membrane technology has gained increasing attention as an alternative to conventional wastewater treatment due to increased urbanization. Among the available membrane technologies, microfiltration (MF) and forward osmosis (FO) have been selected for this study due to their specific characteristics, such as compactness and efficient removal of particles. In this study, two treatment concepts were evaluated with regard to their specific electricity, energy and area demands. Both concepts would fulfil the Swedish discharge demands for small- and medium-sized wastewater treatment plants at full scale: (1) direct MF and (2) direct FO with seawater as the draw solution. The framework of this study is based on a combination of data obtained from bench- and pilot-scale experiments applying direct MF and FO, respectively. Additionally, available complementary data from a Swedish full-scale wastewater treatment plant and the literature were used to evaluate the concepts in depth. The results of this study indicate that both concepts are net positive with respect to electricity and energy, as more biogas can be produced compared to that using conventional wastewater treatment. Furthermore, the specific area demand is significantly reduced. This study demonstrates that municipal wastewater could be treated in a more energy- and area-efficient manner with techniques that are already commercially available and with future membrane technology.

Full-scale in-line hydrolysis and simulation for potential energy and resource savings in activated sludge - a case study

The potential effects of altering primary settlers during biological in-line hydrolysis and converting a nitrifying activated sludge process into a partial pre-denitrification process for the purpose of resource conservation were evaluated. A full-scale primary sludge hydrolysis experiment was performed at a wastewater treatment plant and implemented in a dynamic modelling tool based on ASM2d. The full-scale hydrolysis experiment achieved a volatile fatty acid (VFA) production of 43 g CODHAc·m−3 with no release of ammonium. Additional nitrogen removal of 44 t N·a−1 was simulated, and the produced hydrolysate was able to replace 50% of the annual ethanol usage. Furthermore, 196 MWh of electricity per annum could be saved through the reduction of ethanol production and the optimization of the operation strategy of the activated sludge tank by operating a different number of anoxic zones.

Influences of mechanical pretreatment on the non-biological treatment of municipal wastewater by forward osmosis

ABSTRACT Municipal wastewater treatment involves mechanical, biological and chemical treatment steps for protecting the environment from adverse effects. The biological treatment step consumes the most energy and can create greenhouse gases. This study investigates municipal wastewater treatment without the biological treatment step, including the effects of different pretreatment configurations, for example, direct membrane filtration before forward osmosis. Forward osmosis was tested using raw wastewater and wastewater subjected to different types of mechanical pretreatment, for example, microsieving and microfiltration permeation, as a potential technology for municipal wastewater treatment. Forward osmosis was performed using Aquaporin Inside™ and Hydration Technologies Inc. (HTI) membranes with NaCl as the draw solution. Both types of forward osmosis membranes were tested in parallel for the different types of pretreated feed and evaluated in terms of water flux and solute rejection, that is, biochemical oxygen demand (BOD7) and total and soluble phosphorus contents. The Aquaporin and HTI membranes achieved a stable water flux with rejection rates of more than 96% for BOD7 and total and soluble phosphorus, regardless of the type of mechanical pretreated wastewater considered. This result indicates that forward osmosis membranes can tolerate exposure to municipal waste water and that the permeate can fulfil the Swedish discharge limits.

The effects of physicochemical wastewater treatment operations on forward osmosis

ABSTRACT Raw municipal wastewater from a full-scale wastewater treatment plant was physicochemically pretreated in a large pilot-scale system comprising coagulation, flocculation, microsieve and microfiltration operated in various configurations. The produced microsieve filtrates and microfiltration permeates were then concentrated using forward osmosis (FO). Aquaporin InsideTM FO membranes were used for both the microsieve filtrate and microfiltration permeates, and Hydration Technologies Inc.–thin-film composite membranes for the microfiltration permeate using only NaCl as the draw solution. The FO performance was evaluated in terms of the water flux, water flux decline and solute rejections of biochemical oxygen demand, and total and soluble phosphorus. The obtained results were compared with the results of FO after only mechanical pretreatment. The FO permeates satisfied the Swedish discharge demands for small and medium-sized wastewater treatment plants. The study demonstrates that physicochemical pretreatment can improve the FO water flux by up to 20%. In contrast, the solute rejection decreases significantly compared to the FO-treated wastewater with mechanical pretreatment.