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Anaerobic Digestion Enhancement Success Stories from Around the World

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Rising energy costs and environmental impact are driving the demand for energy from renewable sources. Biomass and wind turbines are the most prevalent form of renewable energy in Europe but the waste water industry also has the opportunity to create electricity from the anaerobic digestion (AD) process. In Great Britain it is thought possible for 2–3% of the country's total energy requirements to be generated from existing AD plants; furthermore if all of Great Britain's AD plants were equipped with Anaerobic digestion enhancement up to 30% more electricity could be produced. The good news for the waste water sector is thatpaybacks are significantly shorter than those of wind turbines.

The Crown anaerobic digestion enhancement system is currently in operation on 20 sites throughout Europe and Australasia and is consistently demonstrating a significant increase in biogas production, reduced solids for disposal, elimination of digester foaming and improved dewatering characteristics.

A number of these projects have been the subject of intense scrutiny and performance testing that has consequently given rise to a significant amount of post installation data. The sites involved cover two countries - Germany and New Zealand. The objective of this paper is to demonstrate the data collected from each of these three sites and compare the results before drawing the conclusions.

The Crown anaerobic digestion enhancement system is frequently referred to as the Crown disintegration system. It is a medium pressure system operating at 10 bar (145 PSI) that mechanically disrupts the cell structure of waste activated sludge prior to digestion in order to improve volatile solids destruction within the digester. Improved VS destruction inevitably leads to increased biogas production and reduced solids for disposal. Biogas increases have been as high as 30% on some sites and the solids for disposal have reduced by as much as 20%.

The Crown system is normally installed between the sludge thickener and the digester and can treat thickened waste activated sludge (WAS) between 3 – 8% dry solids (DS).

The European waste water industry receives credits known as “Renewable Obligation Certificates (ROC's)” for every kW/hr of renewable electricity produced. These in turn are sold to energy generators for approximately twice the standard rate price per kW/hr of generated electricity. The generators need to buy renewable energy as they are tasked with meeting the European Union's target of producing 12% of total energy use from renewable resources by 2010. Under these conditions payback is normally achieved within 2 – 3 years, this compares very favourably with estimated paybacks for wind turbines.

The Crown plant can also be used to release a source of carbon for denitrifying purposes in activated sludge plants as an alternative to the importation of methanol. In addition to the methanol cost savings client also reported a 10% saving due to reduced aeration rates. This paper details the results of a year long study, on a full scale plant installed at Wiesbaden, Germany, that demonstrates a two year payback.
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Keywords: CO2 saving; Crown disintegration; activated sludge; anaerobic digestion enhancement; biogas; carbon sourcing; foaming elimination; hydrolysis; improved dewatering; reduced solids; sludge dewatering; sludge reduction; sludge saving; sludge stability; sludge tanks; sludge treatment; wind turbines

Document Type: Research Article

Publication date: 2009-01-01

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