Sludge and Biosolids Management

Water treatment (except for ion exchange, etc.) involves separating insoluble matter from the water. It could be suspended organic and mineral matter when surface water is clarified for potable or industrial use, or when wastewater is clarified for return to the water environment.

It could be the surplus biomass generated when water is biologically cleaned using micro-organisms to break down organic matter and dissolved constituents. It is called “sludge”, which in English does not conjure positive images, indeed, it sets up negative expectations of dirty, greasy, clinging, stuff that you want to avoid. The equivalent words in French and German are “boues” and “schlamm”, which both mean mud: people take therapeutic schlammbad (mud baths) – sludge baths does not have the same ring. About 15 years ago the term “biosolids” was coined to differentiate sludge that had been treated and was fit for beneficial use on land. Public acceptance might be improved provided use of the term is associated with appropriate good behaviour.

The subject of public and stakeholder acceptance is paradoxical. Everybody recognises that it is the bottom line, but many in the industry restrict themselves to complying with obligatory requirements and some seem wedded to CATNAP (Cheapest Available Technology Narrowly Avoiding Prosecution). To an extent, this is a function of how the industry is financially regulated in the UK, but there is a similar attitude of few being prepared to “go the extra mile” in many countries.

Odour is the factor most likely to draw public outrage; odour is not inevitable and it is not regulated. There are some treatment technologies that produce biosolids with acceptable odour and others that do not. When solid biosolids are land applied there is at least one hectare of treated surface, which is potentially odour generating, between the spreader and the ploughs, even with the best co-ordination. If local residents are outraged by odour they might turn to the web to find out more about sludge/biosolids and some of the sites they find will inflame their outrage.

These sites might not be truthful but they are believable. The paradox is that people continue to apply technologies that achieve the regulated requirements (bacterial counts, etc.) with little or no regard to the odour of the biosolids when they are land-applied.

Commoner (1971) coined the laws of ecology:

  • Everything is connected to everything else
  • Everything must go somewhere
  • Nature knows best
  • There is no such thing as a free lunch

Treating and disposing of sludge

Sludges have to go somewhere. The European Union classifies sludges as wastes in legislation. It has a hierarchy of preferences for treating and disposing wastes. In order of preference, this is: reduce, reuse, recover, dispose with energy recovery and dispose. The hierarchy is for guidance though it is sometimes regarded as a direction. The more we treat water, the more sludge we get, so reduction is not applicable, at least until we come to treatment, and neither is reuse. When choosing a strategy for reduction, use, recovery or disposal it is always a good idea to undertake a failure mode analysis – what could go wrong, how quickly could failure occur, how much warning might we get, how long will it take to put right and what do we do in the mean time?

The properties of indigenous sludge (i.e. that originating at a treatment works) changes during a day and also seasonally; this is in addition to changes in quantity. Sludge settles in sewers during times of low flow, this is then flushed to the works during high flow. Biological activity occurs in sewers, it can be aerobic or anaerobic depending on the gradient etc. If there is a significant “resort” component to the catchment or other seasonal activity (e.g. a food processing campaign) there will be a large difference between low and high season.

The alterations in sludge properties are even greater when a works is a regional sludge reception centre. This might seem fairly obvious but there are many examples of sludge treatment systems that have failed because they have not been able to cope with the changes in the properties of the sludges that they were required to treat.

Oil from sludge was a nice idea but apparently, the main reason that the only operational plant (Subiaco, Perth, Australia, 300,000 pop. equiv.) was abandoned quite soon after commissioning was that the process would only work satisfactorily if the quality of the sludge was relatively constant.

It did not help that the oil that it produced did not match any of the oil industry’s specifications and, since the quantity was small in relation to the quantities that oil companies deal with, it was not worth the industry adapting to this unusual oil. But the bottom line was, the process could not cope with a real-life operational works

Combustion is sometimes portrayed as energy recovery but this is arguable because sludges start out with large initial water contents, which means just getting sludge to an “autothermal” condition is generally a challenge, i.e. getting it to a moisture content where combustion is sustained without requiring additional fuel is difficult. Drying dewatered sludge in an evaporative thermal dryer consumes energy that would otherwise be supplemental fuel in an incinerator. Energy-neutral disposal via combustion has been a reasonable target because of the limitations on mechanical dewatering

Ashbrook Simon-Hartley has an innovation (McLoughlin, 2005) to enhance water removal during belt filter press dewatering by applying an electrical potential gradient across the belts. They call it electrokinetic geosynthetic (EKG*) technology. During the initial development phases, a voltage gradient of 30 volts produced cake in excess of 40% dry solids, with no voltage gradient the cake was only about 20%DS. When this is developed into a practicable dewatering technique, it will have a profound effect on sludge management because dewatering has such an effect on downstream processes, including combustion, which could then truly become energy yielding.

Some power stations, cement kilns, etc. burn sludge along with the conventional fuel. However, the EC has said the legal classification of sludge is waste and if a facility burns waste, it must comply with the Waste Incineration Directive (WID). Cement kilns are willing to reduce their fossil fuel usage by burning other materials such as used tyres and sludge provided it is not detrimental to the quality of the cement, but coal-fired power stations have been reluctant to be brought under the WID. It would seem sensible and proportionate to have similar emission limits, proportionate to risk, for all combustion plants, irrespective of whether they are burning waste, fossil fuel or renewables, though it would be wasteful to spend significant sums on monitoring when there is no risk of exceeding safety thresholds.

Commoner’s laws of ecology appear simple but they bear consideration; it is folly to focus disproportionate effort on one environmental compartment and neglect the consequences on the other compartments and also on the economic and social aspects of sustainable development.

Anaerobic digestion (AD) of sludge

For several years, there has been sustained interest in enhancing the performance of anaerobic digestion (AD) of sludge to increase the yield of methane-rich biogas and to attain greater and more assured reduction of the microbiological indicator organisms. AD has been widely used for sludge for more than 70 years. It is reliable, robust and tolerates variations in sludge characteristics. It is a biological process and therefore susceptible to toxic shock but such events are infrequent nowadays because of the success, established over many years, of controls on discharges from industry and the general restrictions on use of dangerous substances.

The biogas is typically two-thirds methane and the rest is mainly carbon dioxide; the actual proportion depends on the proportion of fat, carbohydrate and protein in the organic matter. For more than 70 years biogas has been used to generate electricity by burning it in adapted engines; the cooling water from the engines has been used to heat the digesters and for space heating (combined heat and power, CHP). Where there are no engines the biogas is used in boilers to heat water. A flare is always needed for excess gas including engine shutdowns. Biogas can be used on site or piped to a more convenient location, for example siting a CHP engine in a village, hospital, industrial area, etc. It has also been used in road vehicles for many years.

The optimum temperatures for AD are mesophilic (35oC) or thermophilic (55oC), abbreviated as MAD and TAnD respectively. MAD is more widespread. TAnD is quicker and causes greater reduction of enteric pathogens but the digestate is probably more difficult to dewater. TAnD used to be thought to be less stable than MAD but apparently this was a perpetuation of faulty research results. TAnD has been implemented successfully at several works in USA.

Modern AD plants aim to maximise the feed solids of the sludge, within the limitations of the ability to mix the digester, and store the biogas away from the digesters, frequently in a double membrane fabric gas holder. They can achieve 67% volatile solids destruction (VS is measured by loss on ignition, it is equivalent to organic matter). Greater VS destruction means the digestate is less smelly and there is more biogas – it is a linear relationship. Digesters with a high “aspect ratio” (height:diameter) are easier to mix. In the USA, digesters typically have low aspect ratios (pancakes) but apparently, this is historic and has no objective basis. Digesters used to have floating roofs which were used as gas holders but there is appreciable loss of gas from the annular sludge surface between the wall and the roof. In a modern biogas facility with CHP raw sludge is worth approximately ¤150/tDS (tonne dry solids) in the UK in income from electricity sales with “Renewable Obligations Certificates”.

AD is performed by two families of bacteria: acidogens and methanogens. Acidogens break down large complex organic molecules into small organic acids and methanogens convert the organic acids into methane and carbon dioxide.

Traditionally these have been expected to co-exist in single mixed digesters but actually they like different conditions. Acidogens work (optimally) at a pH of 5.0 to 5.5 and their growth rate is relatively rapid so they are happy with 1 to 3 days retention whereas methanogens prefer pH 7.5 and grow slowly so they require more than 7 days retention. The hydrolysis and fermentation bacteria produce a small amount of hydrogen, which acidogens use, but which inhibits methanogens.

It is perhaps surprising that people did not start to look at separating the two phases until the 1970s and it has been slow to catch on. Slowly people realised that either by re-configuring their existing digesters, or by adding tanks in front of them, they could achieve a short-retention acid phase separate from the longer retention methanogenic phase. This not only gave greater solids breakdown, less digestate and more gas, but it also gave enhanced destruction of the faecal bacteria that are used as indicators of pathogen die-off.

United Utilities developed pseudo plug-flow acid phase by using six tanks in series (e.g. Mayhew, et al., 2002) and has found differing conditions through the sequence of tanks because of the differences in microbial ecosystems that develop. Plug-flow means greater reduction in faecal bacteria. Thames Water (Asaadi and Marsh, 2005) has also achieved enhanced faecal bacteria reduction, biogas yield and VS destruction but using a single acid phase tank.

Primary sludge digests easily but secondary sludge (particularly from activated sludge) is more difficult particularly because of extra-cellular polymer and cell walls that protect the cell contents from degradation. Bacterial hydrolysis alone is quite slow when working on large sludge flocs. Panter (2006) reflected that there is a 1014 size difference between a 50?m sludge floc and a 0.8nm acetic acid molecule, which is comparable to London and a house brick.

When cells die they release hydrolytic enzymes which cause biochemical hydrolysis. There has been a growth in disintegration technologies to reduce the cities to bricks and release hydrolytic enzymes. Ultrasound has had variable results but there are successful installations that have been operating for several years. The design of the ultrasound horn is a factor and it has been found serendipitously that a buffer tank after sonication has the benefit of allowing time for enzymatic hydrolysis to maximise biogas yield in AD. MicroSludgeTM uses a high pressure homogeniser with alkaline hydrolysis and is under active evaluation. Cambi thermal hydrolysis ( was invented in 1995; there are 10 plants worldwide with more expected in the future.

Feed sludge (dewatered to about 18%DS) is heated to 160oC (6.5 bar pressure) with steam. The combination of pressure-cooking for 30 minutes followed by a flash pressure drop sterilises and disintegrates the feed sludge. Even activated sludge has a high biogas yield following Cambi. The digestate is very easy to dewater; cake in excess of 30%DS from a conventional belt press is typical resulting in considerable reductions in operating costs (Evans, 2003).

The potential of biogas

Biogas from other organic wastes has considerable potential. Denmark has made this part of its national energy strategy and there are more than 20 centralised biogas plants co-digesting a variety of materials and operating CHP (Evans, et al. 2002). The first application of the concept in the UK was at Holsworthy in Devon. The digestate is used on farmland to build soil organic matter and replace mineral fertiliser. It is regarded as an exemplar by the Environment Agency. A cleaner, simpler, less bureaucratic approach to regulation would be very welcome to make this sustainable approach to waste management and recycling easier.

Lime is used to treat sludge at many sites. The capital cost of the equipment is low and it is quick to install. It is very effective in destroying faecal organisms, but the product from the cheapest systems can be smelly and it is worth spending the extra money on a good quality mixer to achieve lime stabilised cake with acceptable odour. The operating cost of lime stabilisation is quite high because of the cost of lime, there is also an issue that some who have selected it because of the already mentioned advantages have not thought about the lime requirement of the surrounding soils.

If soils are calcareous, a bit more lime will not make any difference but if they are neutral, the addition of unnecessary lime can induce trace element imbalances. Sludge & Biosolids Done properly with due regard to “welcomed practice” it is very useful.

Composting converts the labile carbon fraction that AD converts to biogas into CO2; it uses energy in the process and volatilises nitrogen as ammonia. Compost has much less fertiliser replacement value than digestate but it has other benefits. For those who want to make growing media or to sell bagged soil improver to gardeners composting is the preferred route (Evans and Rainbow, 1998).

Drying biosolids

Drying undoubtedly transforms the handleability of biosolids. Evaporative drying on open-air sand-beds went out of fashion in Northern Europe in the 1980s because of the value of the land and the labour input but in climates with more assured drying conditions it can be very effective. It produces a sanitised product. Thermal drying using fuel to evaporate water from dewatered cake is necessarily expensive; the mass of end-product is less than the mass of cake but the break-even between transporting water and evaporating it is quite a long distance.

Granular biosolids are not direct substitutes for granular fertiliser because a) they have only about 10% of the fertiliser nutrient content and b) they seldom have such good flight characteristics. Fertiliser manufacturers have worked hard to optimise the flight characteristics and spreading performance of their products to achieve compatibility with precision farming and tramline widths.

Governments generally acknowledge beneficial use of biosolids on land is the Best Practicable Environmental Option (BPEO). It conserves organic matter and completes nutrient cycles. In addition to agriculture, biosolids are also very valuable in land reclamation; a rule of thumb is that the topsoil should contain at least 2000 kg nitrogen per hectare if there is to be a self-sustaining ecosystem. This will release about 100 kg plant-available nitrogen to plants which enables them to grow and recycle nutrients through their leaf-fall. This is modest by comparison with fertile soils, which contain 5000-20000 kgN/ha.

In any discussion of using biosolids on land there must always be consideration of the hazards which have really acquired the status of persistent myth. The so-called “heavy metals” used to be an issue and they are regulated in developed countries but their concentrations in biosolids have been reduced by cooperative working with metal discharging industries so that they are no longer an issue.

About 500 years ago Paracelsus (1493-1541) wrote: “Dosis facit venenum.” (“The dose makes the poison.”). The relationship between dose and response (effect) is still one of the most fundamental concepts of toxicology (the science of poisons), but when we discuss environmental alarms and chemical health risks it is sometimes forgotten.

Another fundamental that is sometimes overlooked is that for there to be a risk to a receptor there must be a source and a pathway by which a harmful dose is transmitted. There are many organic compounds that find their way into sewers and sludge but repeated surveys of concentrations and evaluations of pathways have concluded that at the concentrations found there is negligible risk to any of the possible receptors (Smith, 2000). Of course one can find persistent organic chemicals where there has been prolonged application of biosolids (they are persistent!) but there is minimal transmission to water or uptake into plants.

Dangerous substances legislation has prohibited chemicals from marketing and use which has reduced the overall environmental input. Evans et al. (1997) monitored soil microbial respiration quotient in a triplicated field experiment studying operational rates of biosolids application under arable and grass. There was no stress response following biosolids application, however a transient response across all treatments (including the control) to selective herbicide demonstrated the sensitivity of the technique and that if there had been a toxic chemical in the biosolids the technique would have found it.


This article has reviewed a broad breadth of issues related to managing sludge and biosolids. They are an inevitable consequence of treating wastewater and their character is affected by the type of wastewater treatment chosen, therefore consideration should be given to the fate of the sludge at the earliest stage in design. This is seldom the case and expensive mistakes have been made as a consequence. If land application for beneficial use is chosen, it should be thought right through to check that the type of land and the way that it is used will sustain a market for the biosolids.

In today’s society where there is a culture of suspicion and easy access to information via the web it is important to “go the extra mile” to earn public acceptance rather than merely aiming to comply with regulations. That is a given, there are no prizes for not breaking the law, it may be necessary to go beyond these minimum requirements to win trust and acceptance.


Asaadi M. and Marsh P. (2005) Acid phase digestion – experience of two stage anaerobic digestion at Swindon STW. Proceedings of the 10th CIWEM AquaEnviro European Biosolids and Biowaste Conference

Commoner, Barry (1971) The closing circle; nature, man, and technology. Knopf, New York. ISBN: 039442350X

Evans, T.D. (2003) Independent review of retrofitting Cambi to MAD. Water Environment Federation 17th Annual Residuals & Biosolids Conference, 19-22 February 2003, Baltimore

Evans, T.D.; Jepsen, S.-E.; Panter, K. P. (2002) A survey of anaerobic digestion in Denmark. 7th CIWEM AquaEnviro European Biosolids & Organic Residuals Conference

Evans, T.D.; Smith, S.R.; Woods V. (1997) Soil microbial biomass – its response to biosolids

Proceedings of the 2nd CIWEM Aqua-Enviro European Biosolids & Organics Residuals Conference

Evans, T. and Rainbow, A. (1998). Wastewater biosolids to garden centre products via composting. Acta Horticulturae no 469, 157-168

Harrison, D.; Brade, C. E.; Le, M. S. (2005) Engineering aspects of mesophilic plug flow advanced digestion facilities. Proceedings of the 10th CIWEM AquaEnviro European Biosolids and Biowaste Conference

McLoughlin P. W. (2005) belt filter press – fact or fiction? Proceedings of the 10th CIWEM AquaEnviro European Biosolids and Biowaste Conference

Mayhew, M.; Le, M. S.; Brade, C. E.; Harrison, D., (2002) ‘Enzymic Hydrolysis – a low cost solution to optimise existing anaerobic sludge digesters’, Proceedings of the 7th CIWEM AquaEnviro European Biosolids and Organic Residuals Conference

Panter, K.P. (2006) Advances in anaerobic digestion. Proceedings of the RSC IChemE Sludge 14 Conference

Smith, S. R. (2000) Are controls on organic contaminants necessary to protect the environment when sewage sludge is used in agriculture? Progress in Environmental Science 2 129-146

Published: 10th Mar 2006 in AWE International