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Energy Prices and Carbon Footprint Cause a Reversal of Raw Sludge Drying in Favour of Advanced Digestion with Thermal Hydrolysis and Cake Recycling in a number of UK Plants

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Northumbrian Water Ltd (NWL) treats the sewage for about 2.6 million people and a further 1.4M population equivalent of industrial wastewater in the North East of England. The project will treat the sludge from 2 million population equivalent. The Bran Sands site near Middlesborough has a WWTP designed to treat municipal sewage and significant industrial effluent. On the same site the existing regional sludge treatment centre (RSTC) was designed to dewater and dry raw sludge from NWL. There are 7 dryers of 5 ton water evaporation capacity - the largest drying site in Europe at the time built. The majority of the input sludge was delivered as liquid raw sludge to the site by ship along the coast. The escalating cost of energy and the green credits available for renewable energy suggested anaerobic digestion was appropriate for NWL. Another consideration was reducing NWL's carbon footprint significantly.

Initially a study was carried out by Ebcor Ltd to review the whole of NWL region and to investigate the conversion of shipping storage tanks in a number of locations to digesters. The study showed that there was an advantage in converting the RSTC to one major digestion centre for the southern half of NWL and converting existing shipping tanks at Howdon, near Newcastle, for the northern half of NWL. The study also showed that there was a major advantage in reducing transport costs for tankers and mothballing the remaining ship by installing a number of local dewatering plants and transporting the majority of the sludge as cake to the RSTC.

The report also recommended a number of advanced digestion technologies that would maximize the green credit payback and minimize digested solids. For the RSTC the recommendation was for Cambi Thermal Hydrolysis (THP) as a pre-treatment.

A contract for building the green field cake reception and digestion center was written with a preference for thermal hydrolysis pre-treatment of sludge and an outline design for the facility (alternates were allowed).

Running parallel, the sludge from RSTC was dewatered and thermally hydrolyzed in a pilot plant and digested in lab digesters during late 2006 and early 2007. The outputs were to confirm the vendor's claims

The key deliverables for the project were: Minimum digestion volume - 300lbs/1000cuft/day load rate/15-20 days HRT; Volatiles conversion 60%, Digested sludge dewatering >30% DS and a pathogen free product. The lab trials demonstrated the loading rate, the dewatering at 32% DS and the pathogen free product. The VSR method was not successful (incorrect measurement of hydrolyzed sludge DS%) but relative biogas production indicated 60% COD conversion at 20 days. The project aims to deliver 4.7 MWs generated. CO2 saving is estimated at 50,000 t CO2.

The economics of the conversion were such that the project had a payback in about 7 years and gives NWL independence from fluctuating energy prices. The contract was awarded in 2007 and the plant is finishing commissioning.

One of the stated advantages of raw dried granules as an energy product has not been realized and the dried product is being used mainly in agriculture. The success of biosolids projects using well dewatered, non smelly cake, Class A EQ Cambi cake, coupled with good agronomic results has made this a product of choice for agriculture, especially on grassland. So the granules have no real advantage and drying becomes optional.

The example from NWL has been taken up by another British Water Company, Welsh Water, which is building two Cambi Thermal Hydrolysis plant to replace 3 thermal dryers at Cardiff, Swansea and Newport (Wales) that were considered too expensive to operate. The NPC and carbon footprint of the conversion are a major driver for the conversion.

Overall these 3 projects account for about 75,000 dry tons per year of sludge converted to new build thermal hydrolysis plants with anaerobic digestion and go a long way towards meeting targets for green energy for these two regional Water Companies.
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Keywords: Biosolids; Drying; Economics; Thermal Hydrolysis

Document Type: Research Article

Publication date: 2010-01-01

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