FEATURE: RESIDUES TO RESOURCES Boosting secondary sludge gas A common challenge for all types of biogas plants, including municipal wastewater treatment plants (WWTP), is how to increase the methane yield from the substrate as, compared to its theoretical potential, a relatively large portion share is not converted to gas in the digestion process. For WWTPs this is of particular significance as a means of both improving the treatment process while reducing operating costs and environmental footprint. THE MAIN OBJECTIVE OF A MUNCIPAL wasterwater treatment plant (WWTP) is the removal of various physical and chemical pollutants and biological pathogens from the water before recirculating or discharge to a recipient. As a substrate sewage sludge is is differentiated according to the treatment process it has been through. Primary sludge is the result of post-primary treatment, which typically consists of mechanical separation of debris along with other physical or chemical processes to remove suspended particles, large and/or dense particles (grit, grease and scum). Secondary or activated sludge is the residue generated from the biological treatment of wastewater after primary treatment whereas mixed sludge is a combination of the two. The Dutch company HoSt BV is one of the larger European developers and suppliers of biogas technology specialising in the digestion of various types of municipal, industrial and agricultural sludge and organic waste residues including so-called Category 2 residues from the meat industry. In 2012 the company started a broader research project with the objective of increasing the methane yield from the conversion of biomass to biogas in the digestion process. Together with the Technical University of Münster, Germany and the University of Twente, the Netherlands, research has been carried out on several types of biomass like secondary sludge from municipal wastewater treatment plants (WWTP), straw, grass and manure with several different technologies; biomass grinding and size reduction, thermal treatment at several temperatures and chemical treatment at temperatures between 30-90 °C. Economy and scalability Thermal treatment at high temperatures (120- 160 °C) and pressure was found to be one of the best pathways to increase the gas production. Facts: Echten WWTP 16 Bioenerg y International No 82, 6-2015 For straw and cow manure (shown) it increases the gas production significantly. Secondary WWTP sludge also has a significantly higher conversion rate after thermal treatment whereas grass did not. In fact, in some experiments the thermal treatment of grass even reduced the gas yield. Demonstrating proof of concept in the laboratory is one thing, ensuring it works at industrial scale and is economically feasible is something else. – For animal manure it is difficult to make thermal treatment feasible because of its low organic content. The additional heat needed would mean using additional gas as fuel eating into the yield gain so there is no real economic benefit. Of course, there can be circumstances where manure is feasible. For example if it needs to be sanitised at high temperature, if the material is first thickened, or if the plant has strong heat integration with other heat sources. But still, the additional gas gain becomes relatively expensive compared to other substrates, explained Herman Teeselink, Director for HoSt BV. Start-up: May 2013 Capacity: 193 000 m³/annum Input: Secondary sludge Digester(s): 1 x 4 100 m³ thermophyllic 1 x 5 500 m³ mesophyllic Post-digester: 1 x 2 000 m³ Reception: 1 x 100 m³ mixing tank 2 x 2.500 m³ silos for receipt CHP: 1 x 600 kWe Digestate treatment: Phospate removal in stuvite reactor Facts: MAP Struvite crystal is a mineral compound made up of magnesium, ammonia and phosphorus (MAP) that under certain conditions precipitate. MAP elements are found in WWTP water due to the composition of wastewater. Phosphorus comes in as a phosphate from dishwasher and laundry detergents. Urea breaks down into ammonia and carbon dioxide (CO₂) in the anaerobic digester and potable water usually contains trace amounts of magnesium that gets concentrated in the anaerobic digester. For straw, the research suggests that biochemical pre-treatment using medium temperature is the best solution and according to Teeselink, the company is building several biogas units with high straw content at the moment. Volume reduction benefit For sewage sludge the situation is different as the additional gas yield is not the only benefit. As most municipal wastewater companies have no means of further onsite residual sludge treatment or disposal they have to pay for sludge removal and disposal, typically to landfill. In the Netherlands the cost of sludge disposal is approximately EUR 40 per tonne. – When our Thermal Pressure Hydrolysis (TPH) pre-treatment is applied, the conversion of organics into gas is increased by between 30- 40 percent. This results not only in more gas, but also in less residual sludge. Furthermore it seems that the ability to dewater the residual sludge post-treatment is improved leading to a further volume reduction, said Teeselink. Pilot-testing In 2013, HoSt together with the municipal wastewater treatment company Waterschap Reest en Wieden commissioned a new municipal sewage sludge digestion plant in Echten, the Netherlands. The Echten plant is a small WWTP. Its main purpose is to receive and treat secondary sludge from other WWTPs and the plant has a capacity to treat 193 000 m³ of secondary sludge per annum. It is the first system in the Netherlands that digests sludge in a two-step digestion process, a thermophyll digester, followed by a mesophyll digester. The plant is also already energy self-sufficient with a 600 kW gas gen-set providing the heat and power needed »
Bioenergy no 6 October 2015
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