Making sense of sludge

Paul Lowe, Nigel Horan and John Hudson1 discuss the implications of new sewage sludge recycling regulations and the landfill directive

It is estimated that each year 90M tonnes of fresh weight farm manures, 3M tonnes of sewage sludge and 7M tones of treated municipal and industrial are recycled to farmland in the UK. With the advent of new regulations for the recycling of sewage sludge and the implementation of the landfill directive, the issue of harnessing technologies that will provide an ‘enhanced product’, from all sectors of the waste industry suitable for recycling to land or other uses has become an urgent consideration.

While there is a willingness to accept the age-old practice of recycling animal manures to farmland, even though sewage sludge has a similar history, the latter is not as acceptable to the public despite the fact the process technologies for sewage sludge are able to effectively reduce the impact of odours and significantly reduce pathogens.

The recycling of unprocessed and processed food waste and organic wastes of industrial origin carry with them a public perception associated with a significant risk more associated with a chemical hazard than disease transmission. In order to develop a range of solutions, without reinventing the wheel, the transfer of knowledge and technologies between the biosolids industry and the waste industry makes a great deal of sense.


The range of technologies used over the years to treat sewage sludge varies from simple storage to the high-temperature high-pressure systems such as supercritical wet oxidation. Some of these technologies have stood the test of time, others have come and gone. There are techniques, which have been ahead of their time, but never reached their full commercial potential, only later to be resurrected as modern solutions to an age-old problem.

Others simply needed investment to turn them into effective processes but sadly in a low-risk industry, time and commitment was not on their side. The application of product standards to the sewage sludge stream in itself generates further challenges to the designers and operators of sludge treatment plants.

This is to be supported by the introduction of in-line Hazard Analysis Critical Control Points (HACCP) monitoring and control. The result is that technologies for the processing of sewage sludge have been advanced to take account of the present and possible future changes to regulation and control.


Mesophilic anaerobic digestion (MAD) is by far the most common process technique used to stabilise sewage sludge and is practised throughout the world. In the EU alone it is estimated there are 36,000 digesters in operation, stabilising around 3.5M dry tonnes per annum (DTA) of sludge. According to the most recent UK survey of sewage sludge disposal activities (1999) 57% of all sludge arisings in the UK was treated by mesophilic anaerobic digestion before land spreading. By the 1960s the standard technique was to provide a two-stage digestion process.

The first stage comprised a heated tank having a retention time of around 30 days and operating at temperatures between 25-35°C. This was followed by a secondary unheated tank of similar design having a retention time between 20-60 days. In reality, very few digesters in the UK were able to operate at temperatures much greater than 25°C. The process was given a new lease of life in the 1970s following work in Severn Trent Water by Noone and Brade.

They went on to show the cost of construction could be reduced by building the reactor tanks using glass lined steel, providing better mixing, better temperature control and operating at 35°C with a retention period as low as 12 days for the primary stage with a minimum of only 15 days for the secondary stage. Such an approach, coupled with better screening of sewage, more effective grit removal systems and pre-thickening of the sludge prior to feeding to the MAD reactor, made MAD cost effective for both large and small works.

In the last few years there has been an increased interest in pasteurisation techniques, as well as in technologies to enhance the digestion process and increase the extent to which organic solids are destroyed. These enhancing technologies are particularly important when the sludge contains a high proportion of surplus activated sludge.

Typical pasteurisation processes include:

  • direct steam injection,
  • hot water heating,
  • subsurface combustion,
  • combustion gas injection,
  • thermal hydrolysis,
  • pre-stage aerobic pasteurisation,
  • electrical heating.
  • Some of the pasteurisation processes discussed above serve a duel purpose, in addition to reducing pathogens they increase the amount of organic matter destroyed and hence gas production. Pre-stage aerobic pasteurisation is claimed to give a 10% increase in the organic and volatile matter destroyed during digestion. Volatile solids destruction in excess of 55% has been reported for the hydrolysis process.

    Ultrasound is a technique which causes disruption of the cells and renders the substrate more available to the anaerobic micro-organisms in the digestion process. Mechanical disintegration gas been demonstrated to have a similar effect. The MicroSludge process is a pressure and chemical pre-treatment process that changes both the rate and extent to which waste activated sludge is degraded in anaerobic digestion.

    While mesophilic digestion has dominated the process dynamics for the past 50 years, attempts have been made to operate the system in the thermophilic range. A number of process configurations which combine thermophilic and mesophilic reactors are under full-scale trials in both the US and Europe.


    Drying technology has been deployed to produce a product in a form that was easily bagged, stored, spread and marketable. Such products required sludge to be dried to more than 90% as granulated or pelletised. The first of these plants to be installed in the UK was at Avonmouth-Bristol by Wessex Water in 1992, followed by a plant at Barnstable by South West Water in 1996.

    These were followed by others, including the Nothumbrian Water installation at Bran Sands, said to be the largest in Europe. In recent years attention has focussed on improving the safety aspects of drying installations with the industry adopting the HSE guidelines for sludge drying plants. Compared with the US composting of sewage sludge in the UK has never been taken seriously. Most of the schemes could best be described as ‘Heath-Robinson’, lacking in good design and capital investment. However techniques, and in particular mechanised systems, are now a well-developed technology with many variations available.

    The question is will it ever become a widely applied process for sewage sludge? Incineration now plays a significant part in the sludge disposal strategies in Severn Trent, Thames, North West and Yorkshire Water. New incinerators have been built using the latest fluidised bed technology and modern gas cleaning techniques. The current installed processing capacity in the UK is around 386,500 dry tonnes/annum.

    Over the period the current UK incinerators have been constructed, air emission standards have changed. Most of the plants have been built to the former west German standards, initially to the German TA Luft 86 standard and later the 17th Bimsch Standard. In the UK the then Regulator HMIP now incorporated into Environment Agency (EA), in 1990, introduced the Process Guidance Note IPR 5/11 (HMSO 1992) and in 2000 the EU introduced the European Waste Incineration Directive 2000/76/EC. This been ratified into English law. Dates have been set by which the more stringent standards of the directive will apply. There are practical challenges involved in meeting the latest more stringent standards of the EU incineration directive.

    The fact these challenges are being met supports the view there is still a case to be made for incineration in any sludge disposal strategy. The biowaste industry should take note that not one single sewage sludge incinerator has failed to secure planning permission in the last decade. It is interesting to note the biosolids industry has in some cases reverted to long-standing techniques for the treatment of sewage sludge.

    Paramount in this has been the use of lime, especially in the treatment of dewatered sludge cake. The past decade has seen lime treatment change from a stop-gap solution to a more sustainable option. It is by far one of the most effective means of achieving pathogen destruction but it leaves a problem if applying high pH sludge to soils. Therefore, the resulting product end-use can be limited by the local soil conditions.


    To imply that the biowaste industry is only just waking up to the requirement for advanced treatment would be wrong. The industry has been concerned for many years as to how best it can change the emphasis from disposal to recycling. UK government agency WRAP aims to increase the level of recycling by creating opportunities for manufacturers to sell added value recycled products.

    The food industry is now the largest employer in the UK manufacturing sector. According to the EA, the food, drink and tobacco sector is the second largest waste producer. The availability of landfill as an acceptable disposal route is diminishing. This is being driven by the application of IPPC to food manufacturing, by the Animal By-Product Regulations, which requires pre-treatment of meat containing waste prior to disposal and the inevitable pressure as 2010 and the first Landfill Directive targets have to be met. Unlike the experience in the biosolids industry, composting has formed the backbone of developments in the biowaste industry over the past decade.

    However, it has been noted UK experience to date has largely been with composting the easiest feedstock. Future demands will require the processing of more challenging feedstock. The majority of compost (680,000 tonnes) has been manufactured as a soil improver, which comprised 561,630 tonnes of soil conditioner (56% of products) and 117,270 tonnes of mulch (12% of products). The remainder of the products totalled just less than 320,000 tonnes (32%). Turf dressings were the smallest product category totalling 6,253 tonnes (1%).

    The data also showed 94,592 tonnes (9%) was manufactured as an ingredient of top/subsoil and 66,856 tonnes (7%) manufactured as a component of growing media. It is clear from the above that composting of organic waste can be said to have been successfully applied to biowastes.

    Unfortunately a number of the plants have suffered from odour problems, nevertheless well-engineered processes have been developed and successfully operated. The process science has been well studied and engineered systems for handling, turning, temperature and moisture control are regarded as state-of-the-art.


    The use of anaerobic technology for the treatment of biowastes can be found in many forms. What is of more interest in the context of biowaste is the use of the technique to treat much more concentrated solids. While the biosolids industry would be cautious about using the technique to treat a solids concentration of more than 10%ds, the biowaste industry has broken this barrier and uses the process to digest much higher concentrations of organic matter.

    These include:

  • solids phase systems that comprise a sealed container or building to hold solid waste, while biologically active percolate is sprayed on to the material to seed the process,
  • high solids systems that are able to treat waste between 20%-40% dry solids concentrations,
  • low-energy systems that used novel mixing techniques requiring very little energy input,
  • lagoon digesters that are simple airtight lagoons with membrane covers and minimal heating and mixing,
  • continuous stirred reactors that are fully mixed tanks operating between 8-15% solids concentration.
  • What is clear is the anaerobic process is very flexible. Many years ago reactors were designed and installed to handle cattle waste and these were very effective. Other reactors have been designed to handle food waste at commercial centres. Such reactors often handle a variety of feed products often without pre-blending and very often on an intermittent feed basis, which matches the bulk delivery system rather than the needs of the reactor.

    Gas production follows the feeding sequence, however, the methane content of the biogas produced can be lower and less consistent than biogas and conversely the carbon dioxide level is higher. Yet the operating parameters of temperature, volatile acids, pH and alkalinity are common to both biosolids and biowaste systems, while ammonia levels are much more variable.

    Advanced pre-treatment using thermal hydrolysis has been applied to biowastes. Pre-sorting at the treatment site is recommended as a general rule. It will be interesting to observe if the current developments in the anaerobic treatment of biosolids using ultrasonic and mechanical disintegration can be effectively applied to biowaste reactors.


    One of the issues the biowaste industry has to face is the varied nature of the feed source. In the case of biosolids the contaminants of grit and rags can be removed, while legislations gives powers to the sewerage undertakers to control discharges to sewer to minimise the impact of heavy metals and other contaminants on the final biosolids disposal route. Biowastes from the food industry may well be contaminated with packaging.

    Biodegradables from municipal solid waste (MSW), despite source separation, are still contaminated with rubbish. Thus the treatment processes rely on mechanical separation prior to the application of a bioprocess. Ultimately, gasification could well be the final technology for these large, centralised units.

    The drying of sewage sludge produces an attractive product that is acceptable to the recycling industry. The anaerobic treatment of food wastes make is suitable to enter a sludge drying facility. The digested product can be dewatered before processing in drying units. The outcome is an almost identical product to dried sludge pellets with a similar appearance and the same handling properties.

    As a result, the use of the drying adds further value to the gate fee for the raw waste material. Combining the anaerobic treatment with CHP plants and waste heat recovery improves the overall efficiency of the flow sheet. The technologies for the treatment and disposal of biosolids are well advanced. Of all the techniques anaerobic treatment is by far the one most deployed. The reaction kinetics are well understood for thin sludges but the Biowaste industry has shown that digesting highly concentration solids is feasible. What the Biowaste industry can learn from the biosolids industry is that loading rates, mixing and temperature control are critical to a successful operation.

    They can also learn the anaerobic reactor is not primarily a pathogen reduction technique but rather a means of extracting energy in the form of gas and a means of reducing the impact of odours on the local community. For pathogen, reduction time/temperature techniques such as preheating and drying are essential process stages. Perhaps the greatest synergy would be to combine the techniques in jointly operated units where the biosolids reactors can serve as a anaerobic reactor producing the essential micro-organism for biowaste digestion and in so doing provide a more stable reactor and improved gas quality.

    The combination of anaerobic reactors with CHP and the utilisation of waste heat from CHP plants as the heat source for drying plants give the added potential for an economic solution to the biowaste treatment. It has already been shown techniques such as thermal hydrolysis can be deployed in both the biowaste and biosolids flow sheets.

    The opportunity for other techniques to cross the divide must exist. Incineration is still an ideal technique for energy recovery and conversion in power generation plants. It suffers from a poor public image, especially in the MSW field. However, the sewage industry can boast of the fact that modern incinerators can be designed and operated within the strict emission standards of today and with careful management the planning approval stage may be safely negotiated

    1 Paul Lowe works for Aqua Enviro Consultancy Services, Nigel Horan works for Reader in Public Health Engineering for the University of Leeds and John Hudson is an independent consultant.
    Department of Environment, Process Guidance Note RPI 5/11, Waste Disposal and Recycling, Sewage Sludge Incineration, HMSO, 1992.
    GP Noone and CE Brade, Low Cost Provision of Anaerobic Digestion: II. High Rate and Prefabricated Systems. Wat.Pollut.Control, 81, (4), 479-510, 1982.
    CEC: Council of European Communities Council Directive on the protection of the environment, and in particular soil, when sewage sludge is used in agriculture, (86/278/EEC), 1986.
    P Lowe and NJ Horan, Proceedings of the 9th European Biosolids and Biowaste Conference, Wakefield UK, Aqua Enviro, November 2004.
    l This article is a synopsis of a paper presented at the Annual European 9th European Biosolids and Biowaste Conference, Wakefield, November 14-17, 2004.

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