Cutting costs of WATER WORKS

As civil structures go, this is one of the fastest fabrications Graham Clough of Thames Water has come across. `This level of construction progress is quite phenomenal,' he says. As water treatment works have been known to take years from contract to commissioning, his satisfaction with the progress is warranted. `It's been a very interesting build scenario,' says Peter McLoughlin of Simon-Hartley, the Stoke-on-Trent-based contractors, adding, `We built the plant outdoors to avoid planning permission, there are no buildings!'

Detailed design and procurement of the new sludge treatment plant at Thames Water's Chertsey works in Surrey began in May 1998. This time last year PE reported (July/August p.19) on the beginning of its construction. The plant was ready for commissioning in January and is now up and running, producing 8000 dry tonnes of biosolid per year, twice its previous production rate.

A joint venture between sludge dewatering specialists Simon-Hartley and Norwegian company Cambi A/S was assembled to promote a new thermal hydrolysis technology in the UK and Ireland. The Chertsey site is the first such demonstration of this innovative technology.

The Cambi process is a sludge and organic waste water treatment process pioneered in Norway. It was originally developed to combat odour at treatment plants but many additional benefits of the process were also discovered.

Following the European Directive banning disposal of sludge in seas around the UK and Europe, water companies are faced with treating large quantities of sludge and handling its disposal. This completely different approach goes some way to finding a solution to sludge disposal and increasing dewatering capabilities.

The breakdown of sludge

Sewage sludge, often referred to as biosolid, is the by-product of a variation of physical and biological treatments of waste water before it reaches accepted cleanliness levels and reintroduced to the watercourse. Sewage sludge itself is around 96 per cent water which must be removed and sterilised. Recycling of sludge as fertiliser has long been recognised as the most environmentally acceptable option but has come under pressure with tighter controls on its microbial activity before disposal.

Thermal hydrolysis literally means the splitting of cells (lysis) in the presence of water (hydro) by the application of heat (thermal). The sludge is heated in a pressure vessel, splitting the tough cell membranes of micro-organisms present, releasing the long chain molecules within and causing complete sterilisation and pathogen destruction. From there it enters the digesters for final breakdown of solids and water separation. After digestion, sludge can be dewatered to up to 50 per cent dissolved solids and this produces pathogen free cake for land reclamation and agriculture.

Breaking down cell walls using thermal hydrolysis actually aids dewatering by allowing water to escape cell boundaries. A further benefit of this is that less final drying is required, using less fuel gas.

The use of heat treatment in dewatering sludge has, until recently, had a bad reputation. Original tests carried out in the early seventies concluded that despite giving a dramatic improvement, the process created refractory COD compounds. Other disadvantages were strong odour problems and corrosion. These tests were, however, carried out at extreme temperatures above 180 degrees C.

`We actually cook the sludge at temperatures up to 165 degrees C at 8bar,' says McLoughlin. `Cooked food is easier to digest. It comes out as soup and then introduced to digesters. Because cell structure is broken apart it's also easier to dewater.'

Do not disturb

The process ensures methanogenic bacteria in the digesters are not disturbed by any bacterial acivity within the incoming sludge. Consequently the methanogenic bacteria produce more methane than otherwise to provide a net energy profit. Some of the methane gas is used to produce steam to heat the thermal hydrolysis process itself, maintain optimum temperature of the digesters and fuel the sludge dryers.

`After the mass transfer and energy balance equations, you realise the process is more efficient overall,' says McLoughlin. `Plans for a combined heat and power plant (CHP) are underway and we hope to reach 350kW excess production.'

Costing £4.5million, the Cambi process at Chertsey starts with sludge held in a buffer silo before passing to the hydrolysis phase. From there, sludge is fed to the pulper tank where water and exhausted steam from later elements of the process pre-heat and dilute the sludge to produce a feed stock. The sludge is then pumped into a reactor vessel where pressurised steam raises the system temperature to 160 degrees C at 8bar. After a 30 minute detention period, the reactor contents are ejected into a flash tank where pressure and temperature are reduced and waste steam recycled back into the pulper. Storage chambers at each end of the process allow batches on condition that the sludge maintains the correct residence time in the pressure cooker. The whole Cambi process is semi-automated using a partially manned SCADA station.

The overall benefits are four-fold. Temperature, pressure and retention period ensure a fully disinfected product, surpassing all current and proposed regulations for recycling of biosolids to agriculture. Sterilised sludge allows normal digester systems to be loaded at more than double conventional rates, increasing the capacity of existing plants or minimising capital for new works. Methane production is doubled, making the plant self sufficient, with additional surplus for CHP generation. Thermally hydrolysed and digested sludge dewaters to levels 50 per cent greater than conventional methods.

Proud of his product and hopeful for the future, David Davis, marketing manager for Simon-Hartley announces, `In a couple of months we hope to sign contracts on two similar projects coming up, one double and one four times the size of this one.' PE