Cambi AS with its Thermal Hydrolysis Process (THP) has been awarded a contract by Interserve Project Services Ltd for the key component of an innovative digestion project at a large water company in London.
Currently the water company pump raw sludge from a water treatment plant in Essex to a water treatment plant in East London for incineration in conjunction with the sludge from Essex. This plant serves 3.3 million people. Pressure to treat sewage to higher standards has led to a higher quantity of secondary sludge which does not dewater well. This has led to under-capacity at their existing fluidised bed incinerator plant as there is too much water in the sludge.
The project entails renovating abandoned digesters in Essex and treating the indigenous sludge from this plant along with secondary sludge pumped from the East London plant. The outputs will be 4 MW of renewable electricity, a high dry solids pasteurized (Class A) cake for agriculture, and reduced pressure on the existing incinerator plant in East London.
The main project comprises of reception of secondary sludge from East London through the reversed pipeline and blending with indigenous sludge followed by pre-dewatering of the sludge to make a cake of about 18% dry solids. This is fed into Cambi’s THP in 2 streams of each 4 reactors that convert the sludge by the addition of steam to make a disintegrated and sterilized digester feed at about 10% dry solids that will be fed to the existing 4 x 5000 m3 digesters. The digester mixing will be refurbished as part of the main project and the digested sludge fed to new dewatering belt presses. The final biosolids will be used in agriculture. The biogas will be used to produce about 4 MWs of renewable electricity and waste heat recycled to steam for Cambi THP and other uses.
Cambi’s THP, a Norwegian technology, has a strong presence in the UK and Ireland with nine THP plants built, or in design, for about 8 million population. The projects have demonstrated the ability to feed digesters at 2 -3 times conventional load and to make a high quality cake fertiliser product. Other Cambi projects around the world such as Brisbane and Brussels are unique in the ability to digest and dewater secondary sludge only, at large scale; therefore a high load of mainly secondary sludge can be digested in the existing digesters at the plant in Essex.
Generally there has been a trend for more land application in the UK with a move away from incineration as the development of low volume pasteurised products has ensured good acceptance by the farming and food community.
Summing up from a carbon perspective, this project has multiple benefits. Diverting secondary sludge from incineration not only reduces the contractors operating costs it also significantly reduces the carbon footprint because less water will be incinerated. Instead the secondary sludge treated in Essex will supply both renewable power as a stable base load to the grid and it will secure better recycling of valuable resource to the agriculture, which also has a carbon upside as less industrial fertilizers will be used.