Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/867569
SEPTEMBER 2017 WET NEWS 21 INSIGHT Pumps & mixing E nergy generation from the anaerobic digestion (AD) of sewage sludge has reached 888GWh, according to figures from the 2016 Digest of UK Energy Statistics – an increase of over 25% since 2011. Yet over the same period the installed theoretical MWe capacity has grown by just 9%, with only 159 biogas plants using sewage sludge as a feedstock. How has the energy generated by the anaerobic digestion of sewage sludge increased so dramatically, and is the trend for efficiency improvements in the water sector set to continue? One of the biggest changes to the treatment of sewage sludge in recent years has been the use of high temperature heat treatment on sludge. The process sterilises the sludge and changes its rheological proper- ties, making it more bio- degradable and enabling the loading rates to the anaerobic THE CONCEPT • Progressive cavity pumps can maintain flow even with varying pressures and variable sludge viscosities • The pumps' good discharge pressure capabilities enable long distance pumping • The pumps have a different principle of operation, and employ a rotary action via a series of cavities • PC pumps have lower operating and maintenance costs NEED TO KNOW 1 The pumps' non-pulsating flow characteristics make them a popular choice for wastewater applications 2 Raw sewage can constitute up to 99% water 3 Pumps are vital to the water sector's continued success and a 'one size fits all' approach should not be applied 4 Thickened sludge is often dewatered at the point of production using a variety of equipment technologies THE VERDICT "…progressive cavity pumps compare well to other pump technologies with their ability to maintain flow even with varying pressures and variable sludge viscosities, which is specifically important when feeding heat exchangers to ensure the most efficient heat transfer for heating the sludge" Lesley Eaton, Seepex digesters to be doubled, delivering a higher gas yield. A further benefit of the process is that the final sludge from the digester is pathogen free and can be easily dewatered, resulting in a more valuable final product for application to land. Economies of scale Currently, a significant proportion of sewage sludge is treated by water authorities in their own large, centralised treatment centres, which take dewatered sludge from their smaller satellite treatment works. Raw sewage can constitute up to 99% water and the satellite sewage treatment processes work by removing this water and thickening the sludge using a series of steps involving both gravity settlement for primary effluent and mechanical thickening of secondary sludge. Thickened sludge is o—en dewatered at the point of production using a variety of equipment techno- logies to produce sludge with dry solids content (ds%) of between 18-35% before transfer to the larger centres. The process of dewatering cuts the sludge volume and reduces transport costs – vital if economies of scale are to be realised. When dewatered sludge is received at a central processing plant it is mixed with indigenous sludge to produce the ideal ds% for energy generation and other onward processes. However, the trend for water companies to treat only their own sludge is set to change as a result of the Water 2020 report, published by regulator Ofwat, which is designed to kickstart a market for treated sewage sludge. Speaking at last year's UK AD & Biogas Conference, Ofwat principal engineer Alison Fergusson explained that the regulator hopes to encourage markets between operators in order to move sludge from a company where it's expensive to treat, to one that is cheaper. Ofwat modelling suggests that if its proposed changes are implemented, around 30% of all sludge could move between different water companies. The plans aim to make sludge a potentially valuable product, leading to more efficient treatment and greater energy generation. While process optimisation, economies of scale and deregulation will all play a part in the continued increase in energy generation by the water industry, it is important not to overlook the importance of correct equipment selection. Pumps in particular are vital to the water sector's continued success and a 'one size fits all' approach should not be applied. For all pumping applications, the correct pump selection will • Efficiency in action With the UK sewage sludge sector booming, it is vital that the pump is not treated as a standalone commodity. Seepex's Lesley Eaton explains. Anglian Water installed Seepex PC pumps for efficient sludge transfer and mixing duties.