Sludge Treatment
Why do we treat sludge?
All methods of sewage treatment generate organic sludges (or ‘biosolids’) as by-products and these must be managed separately from the liquid sewage (Reference 2). Raw (untreated) sludges have a very high oxygen demand and must not be allowed to enter the water environment.
All methods of sewage treatment generate organic sludges (or ‘biosolids’) as by-products and these must be managed separately from the liquid sewage (Reference 2). Raw (untreated) sludges have a very high oxygen demand and must not be allowed to enter the water environment. Sludge also contains pathogenic organisms.
There is, therefore, a need to deal with them in a way that permits their ultimate disposal in an environmentally acceptable and sustainable manner. The sludge ‘route’ selected for a given sewage treatment works will depend on several factors including its location, the availability of suitable farm land, the characteristics of the sludge and the overall cost.
What does a typical sludge contain?
Sludges produced by sewage treatment are organic in nature and contain useful amounts of plant nutrients such as nitrogen, phosphorus and essential trace elements.
Therefore, the first objective should be to utilise the sludge as a fertiliser or soil conditioner on agricultural land. In fact, some 60 per cent of the sludge produced in the UK is (after appropriate processing) recycled to farms. Agricultural use of sludge is regulated by government controls and by codes of practice designed to protect the quality of the soil, its crops and the health of human and animal consumers of such crops (References 3 and 4).
The location of some sewage works is unsuitable for the sludges they produce to be used in agriculture (e.g. not enough farmland or too much manure from farm animals) and for these the most frequent ‘route’ is incineration with landfilling of the ash. Landfill is an alternative but sludge is seldom landfilled in the UK. Either of these options is regulated strictly in the UK to minimise environmental impact. While these two options account for most of the remaining 40 per cent of UK sludges, there are also other minor uses for sludge, for example as a garden fertiliser, to make compost or as a fertiliser for crops which are subsequently used as fuel at power generating stations.
In their initial form, most raw (untreated) sludges have a high water content (96-99%), are putrescent and have an offensive odour. They will also contain a variety of human and animal pathogens derived from the contributing population.
Various forms of treatment may be used to achieve volume reduction by removing some of the water content. Odour and pathogen reduction is achieved by stabilisation and disinfection processes. In recent years, the control of odour emissions to the atmosphere has become an important requirement of sludge treatment.
The following outlines the more common types of sludge treatment employed, of which various combinations are used according to the end product required.
Primary consolidation
As a first stage of treatment, sludge is passed through stirred tanks or subjected to centrifugation to reduce its water content and volume by up to 50 per cent. The separated liquor is returned to the sewage flow for treatment and the consolidated sludge passes forward for further processing.
Anaerobic digestion
Anaerobic digestion (AD) has been practised for more than 150 years. It is not new but still has huge unrealised potential if regulations did not inhibit co-digestion of wastes. In this process, consolidated liquid sludge is retained in an airtight tank (digester) and maintained at 35 deg. C for 12-20 days. Under the anaerobic conditions in the tank, various pathogens break down about half of the sludge organic matter and convert it into a gas containing about 65 per cent methane.
The gas is used to heat the digester and, in some cases, also to fuel gas engines to generate electricity. The sludge resulting from anaerobic digestion is much less offensive in odour than the untreated raw sludge and, with certain restrictions (Reference 2), is generally suitable for use in agriculture in liquid or solid form. Further consolidation of sludge after digestion, to reduce its volume, is a common practice.
Mechanical dewatering
Either untreated or digested sludge may be converted from a liquid to a sludge ‘cake’ by treating it first with a conditioning chemical which releases much of the water initially bound to the organic matter. Much of the free water is then removed from the sludge in a filter press, a belt filter or a centrifuge. The resultant sludge cake will have only 20 per cent of the volume and weight of the original sludge, thereby reducing subsequent handling and transport costs. The conversion of sludge to a solid form is essential prior to its disposal to landfill.
Incineration
This involves the burning of sludge at 850-900 deg. C to destroy its organic content and to leave a smaller residue of mineral ash for final disposal, usually to landfill. Incineration is only suitable for large sewage works and is used when the option of agricultural use of the sludge is not practicable. The process is carried out under closely controlled conditions and is subject to strict environmental regulation (the EU Waste Incineration Directive) to ensure that ambient air quality is not compromised by the combustion gases.
Thermal drying
Some sewage works in the UK employ thermal drying systems to convert the sludge to pelletised or granular form comprising about 90 per cent solids. The heating involved also destroys pathogens. Thermally-dried sludges are used in agriculture or for amenity uses (for example, golf courses, parks and other amenity areas).
Pasteurisation (disinfection)
To destroy all pathogens in liquid sludge, it is heated to about 70 deg.C for at least 30, minutes after which it is cooled and subjected to anaerobic digestion. This combination of pasteurisation and digestion produces an ‘enhanced treated’ product (Reference 4) which enables it to be used more widely for various agricultural purposes.
Lime stabilisation
At some smaller works, lime is added to liquid sludge to raise its pH to above 12.0 for several hours. The high alkalinity improves its odour and eliminates pathogens.
Composting
A few sewage works compost sludge by the process of ‘windrowing’. The process generates heat and a rise in temperature in the composting material causes pathogen destruction. The final product may be suitable for amenity use.
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