Garbutt, S.1, Dutt, S.1, Martin, J.1, Rawlinson, D.2, 1Aker Solutions, 2Northumbrian Water Ltd(free)
In August 2009 Aker Solutions began commissioning three advanced digesters at Northumbrian Waters’ Bran Sands Site. The digesters were built as part of a design and construct contract awarded in June 2007.
Three digesters, each of 6700m3 volume, were designed to process up to 40,000 tonnes of sludge dry solids per annum. The digesters produce biogas which is used to generate 4.7MW of electricity for use on the Bran Sands site. The stabilised sludge solids from the digester are dewatered to produce a Class A biosolids product.
This paper covers the start-up period for digester seeding and initial operation. It includes the general seeding methodology, problems experienced, remedies found and lessons learnt. Laboratory test results from sludge samples taken from throughout the start-up and commissioning period are presented to illustrate the lessons learnt.
Keywords: Commissioning, Digester, pH, Seeding, Volatile Fatty Acids
Aker Solutions was awarded a design and construct contract by Nothumbrian Water Limited (NWL) in June 2007. The contract was for a new Advanced Digestion (AD) and Combined Heat and Power Generation Plant (CHP) at their existing sludge treatment facility at Bran Sands, Teesside. The plant was designed to treat 40,000 tonnes of sludge dry solids per annum and generate up to 4.7MW of electrical power for use on the Bran Sands site. In addition, the treated biosolids produced would be a class A biosolids product and could be sold to agricultural users.
This paper provides a summary of the commissioning experiences and lessons learnt during the start-up, seeding and commissioning of the Bran Sands advanced digesters.
The aim of the project was to significantly reduce the operational expenditure of the existing site and provide a sustainable plant and process. The existing plant used natural gas fired dryers to dry dewatered sludge cake before being pelletised. Natural gas was also used in gas turbines to generate electricity for use on site. The new AD plant would reduce OPEX by reducing the natural gas and imported electricity consumption of the site. The economic analysis for the AD project was based on an equivalent of 40,000 tonnes of dry sludge solids per annum. Prior to the construction of the AD plant NWL were using up to 2.0 MW of electrical energy and 14 MW of energy in the form of natural gas.
Other benefits of the construction of the AD plant include:
• A reduction in CO2 emissions and reduced consumption of fossil fuels from the drying process;
• A sustainable source of electricity; electricity is generated under the Renewables Obligation Certificate Scheme;
• Production of a class A biosolids product which can be used as a fertiliser and reduces the need to landfill or incinerate the sludge which would produce even more CO2;
• The process and treated product has negligible odour;
• Significant operational cost savings;
• The design utilises existing assets where possible; and
• A move towards self sufficient sewage sludge treatment.
Following contract award, Aker Solutions undertook a design process which included design specification, supplier selection, integration of vendor packages into the design, project management, HAZOP, SIL review, conducting an operational research model (ORM) and availability, reliability and maintainability study (ARM) and pollution prevention and control (PPC) application. Northumbrian Water and Aker Solutions with the support of Entec and MWH worked closely together during the detailed design phase of the project.
Construction commenced in September 2007 with pre-commissioning activities carried out in early 2009. Functional testing (on water) was completed in April 2009. After lining of the digesters was completed, process commissioning (with sludge) began in August 2009. The first seed sludge load arrived on-site in early September 2009 and power has been produced on an ongoing basis from the end of October 2009. Contract completion was achieved in February 2010.
For the purposes of commissioning, Aker Solutions’ process, mechanical and electrical, instrumentation and controls commissioning engineers were supported by a dedicated team of Northumbrian Water operatives. Northumbrian Water also contributed a team of acceptance engineers to ensure the plant met performance criteria and standards. The close working relationship between Northumbrian Water and Aker Solutions’ commissioning personnel was key to the successful completion of this project.