The impact of tightening wastewater treatment drivers on sludge processing and its carbon impact

Around the world, the production of sewage sludge is increasing. This is due to a growing population, movement of a fraction of this population to cities being connected by new sew age treatment facilities (as occurring in China), and also to a tightening of environmental regulation surrounding wastewater treatment. A study in the UK found that implementation of stricter environmental standards had a negative impact on energy consumption and consequent carbon impact. Reducing the BOD in the final effluent from a figure of 25 to 5 mg/l increased the energy required by the process by three times. The UK Environment Agency recently released a report saying that implementation of legislation like the European Union’s Urban Waste Water Treatment Directive (UWWTD – 91/271/EEC) and the Water Framew ork Directive (WFD - 2000/60/EC) would increase the carbon footprint of the entire Water industry by nearly 3%. It further concluded that the carbon impact of several works w ould have to double to meet new wastewater drivers. The Water Industry is a large consumer of power and subsequently a large generator of green house gas emissions. In the UK, the Water Industry carbon footprint accounts for over 1% of the nations emissions and is only second to the power industry. Therefore, any legislation which could increase the impact of the Water Industry’s carbon footprint has an inherent influence on a nation’s carbon emissions. A trade-off therefore exists between improving the local environment w hilst minimising impacts at a global scale.

In Europe, the implementation stricter aquatic standards, is resulting in configuration changes in wastewater treatment. As facilities upgrade to total nitrogen removal, primary sludge is often being used as a supplemental carbon source for denitrification. Not only does this remove easily biodegradable BOD from the sludge treatment train, it also decreases (or eliminates) the quantity of primary sludge produced as it is replaced by an increasing concentration of secondary sludge. Additionally, upgrading secondary treatment from carbon removal to nitrification results in increasingly extended aeration times during secondary treatment. Whilst it is well known that secondary sludge digests far poorer than that produced during primary treatment, recent work has shown that its digestibility degrades further as aeration time increases. Furthermore, phosphorous removal, using chemicals such as iron or alum, also generates larger quantities of sewage sludge but with decreasing calorific value. In summary, not only do tightening wastewater standards increase energy consumption in their acquisition, but they are also influencing the production and type of sew age sludge being produced. This sewage sludge is becoming increasingly difficult to process resulting in additional negative impacts on: renewable energy generation via digestion; dewatering and thermal processing. This paper highlights the results of modelling w ork w hich describes how the production of sludge alters with differing wastewater drivers, and w hat the potential impacts are for downstream processing of the sludge and its influence on carbon footprint.