Energy boost

Mogden Sewage Treatment Works in South London, which is designed to process flows of 810Mld for a local population of 1.8 million, has seen two major events that have driven the need to improve its sludge handling and treatment.

To take account of legislation changes regarding the use of sewage sludge in agricultural applications, Thames Water Utilities entered into an alliance with Black & Veatch to upgrade the site's sludge processing systems. The result is improved sludge thickening and digestion.

In addition to satisfying the two drivers, these improved sludge processes have resulted in an increase of biogas production. For Mogden, this is a resource, rather than a by-product - the gas is used to generate energy in on-site CHP engines and a boiler plant. The increase has also resulted in a predicted substantial saving in imported electrical energy. The use of the biogas for on-site electrical generation has also attracted Renewable Obligation Certificates (Green Certificates).

Mogden STW, which was built in the 1930s, processes sludge containing organic matter, grit, sand and quantities of rags. This is taken from on-site processes (primary, secondary and final tanks) and from imported sludge delivered by tanker from a variety of locations. To ensure compatibility with the sludge produced on site, the imported sludge passes through a reception plant consisting of a storage tank and 6mm fine screening. The plant is designed for a sludge feed dry solids content of 7%.

<b>Digester refurbishment</b>

Mogden has 20 digesters, each with a working volume of 4100m3. Of these, 16 have been refurbished as digesters, two converted to emergency storage, and two decommissioned.

There has been a history of reduced digestion capacity at Mogden, due to grit and screening passing through the works and accumulating in the digester tanks. However, a significant improvement in grit removal and the provision of 6mm fine screens under a previous Black & Veatch project has reduced grit and screenings carry-over. The digester tanks were cleaned prior to refurbishment, and two years later are still operating at full capacity.

Refurbishing the digesters involved installing dedicated heat exchangers and a gas mixing system for each of the tanks. The previous plant was internal and inaccessible, but all the new equipment is outside the tanks. The system comprises a heat exchanger which transfers a minimum of 638kW of heat to the sludge. Each digester has a pump to circulate both heating water and sludge through the heat exchanger. The temperature of the sludge is maintained by a modulating three-way valve in the heating water circuit, which controls the heating water flow through the heat exchanger.

<b>Rolling boil</b>

The gas mixing system uses a compressor, which draws gas from the bell of the digester and delivers it into the sludge through solenoid-controlled nozzles. The nozzle layout and solenoid operating sequence produces a 'rolling boil' within the sludge, which mixes the feed sludge completely into the digester volume within two hours from introduction. A 3MW dual fuel boiler, associated heat exchanger and up-rated sludge feed pumps pre-heat the sludge feed to the digesters. The boiler can operate on either biogas or fuel oil, with automatic changeover from gas to oil in the event of low gas pressure.

Mogden STW has four existing dual fuel Mirlees CHP engines, each capable of delivering 2.4MWe. Previously, 1.3MW of power was recovered from the exhaust gases, from a total of around 2MW recovered heat.

To improve the heating to the digesters, the exhaust gas waste heat boilers were replaced and upgraded to provide increased heat recovery from 1.3MW to 1.9MW, giving a maximum heat recovery from each engine of 2.6MW.

As part of the heat recovery from the engines, the circulation water provides cooling for the engines, removing heat from the turbo inter-cooler, valve cage and jacket water. This cooling load results in more heat in the circulation water than is required by the digestion process during the summer months. Previously, this heat would accumulate in the water circuit, leading to engine overheating and consequential failure. To avoid this problem, radiators, rated at a total of 2.6MW, have been provided to dump excess heat to the atmosphere. These allow excess heat to dissipate, and reduce outgoing pressure to the digester heating system when all digester heating is satisfied. A three-way valve, modulated subject to heat, controls the flow through the radiators and the pressure conditions within the circulation water loop.

The gas mixing and sludge heating of each digester and the in-line feed sludge heating boiler are controlled by individual control loops. All are controlled by a distributed control system consisting of PLCs contained in RTUs. Plant and process status and condition are available for view at the site control room and at the RTUs.

The overall control system is, in fact, one of the most significant changes to the plant. It monitors and manages the heat recovery, distribution and utilisation between the CHP engines and the digesters. Problems associated with digesters running too cool, overheating engines and uncontrolled system pressures are now a thing of the past.

Although not directly related to the sludge process improvements, the beneficial spin-off from the increased gas production is the ability to operate the CHP engines on gas for longer periods. As a renewable source of energy, the use of biogas as the major fuel in the engines has attracted 'Green Certificates'. Arising from a new tariff structure for electrical consumption, Thames Water Operations has recently employed Black & Veatch to reconfigure the site's HV distribution to take advantage of the 'Embedded Generation' at Mogden STW during the next 12 months.

<b>Green Certificates</b>

Renewable Obligation Certificates, also known as Green Certificates, are part of the UK government's strategy to promote the generation of electricity from renewable sources. Awarded by the energy regulator, Ofgem, each certificate shows that 1MWh of power has been generated from a qualified renewable source. These sources include wind turbines, solar power generators, wave generators, and biogas generation.

All energy suppliers in the UK are obliged under the Utilities Act 2000 to purchase an increasing percentage of the energy that they supply to the National Grid each year from renewable sources. They do this by presenting the ROCs for the energy to Ofgem; if they cannot purchase enough, Ofgem can sell them a 'buyout certificate'.

The proceeds from these are 'recycled' to the companies which generated the power for the ROCs and cashed them in. ROCs can themselves be traded, and plants in England, Scotland and Wales qualify for the scheme.