|Urban Wastewater Projects - A Layperson's Guide (EEA, 1998, 124 p.)|
|Chapter 2. An Introduction to Urban and Rural Wastewater Management|
2.8.1 What is wastewater sludge?
These solid residues are separated from the watery effluent from the process either by physical screening and filtration or through sedimentation. Because these processes take place in a watery environment and it is difficult to remove all of the water, the residues are not dry and, in fact, generally contain a high proportion of water prior to their further treatment. This mixture of solids and water is termed sludge.
Sludge is, in effect, a concentration of the polluting material in the wastewater, either in its original form or transformed by the treatment processes. It has a high organic content and, if not treated, rapidly putrefies and gives off objectionable odours. It is therefore subjected to a series of treatments in order to stabilise it and to remove sufficient water from it to enable it to be disposed of without its causing nuisance or polluting the environment.
Depending upon the degree of stabilisation and dewatering required, facilities for the treatment of sludge can cost between 30% and 50% of the cost of the treatment plant.
Sludge is more viscous than sewage and its treatment often poses more problems in operation than wastewater treatment. However, it is essential to appreciate that:
· no treatment plant can operate without producing sludge at some time and normally it is produced in significant quantities every day
· no treatment plant should be built without ensuring that there is a secure means of disposing of the sludge
· every wastewater treatment plant must be provided with sludge treatment processing facilities capable of adequately preparing the sludge for the disposal option chosen.
Sewage sludge utilisation is regulated by the EU Directive 86/278/EEC. The Urban Wastewater Directive 91/271/EEC prohibits marine disposal of sludge after 1998. However it should be noted that several countries have much stricter national regulations.
Fig. 2.6 Liquid sludge from wastewater treatment process is stabilised and dewatered prior to disposal or beneficial use
2.8.2 What are the disposal options for wastewater sludge?
There are not many options for the disposal of sludge. The two most common are disposal into a sanitary landfill and the beneficial use of sludge in agriculture, horticulture and forestry.
Disposal into a sanitary landfill can either be as dewatered sludge or, following incineration, as ash. When disposed into landfill, an especially prepared area, rendered impermeable, should be used, if it is necessary to protect the groundwater below the site from pollution. Surface water runoff and drainage from the sludge from this area will need to be subjected to treatment.
The use of sludge in agriculture is the preferable alternative. However it should be appreciated that the sludge can contain disease-causing bacteria, sometimes in a cyst form resistant to disinfection, as well as organisms that can harm certain crops. It may well be necessary to carry out some form of pasteurisation of the sludge or long-term storage to avoid harmful effects. There are a number of processes that convert the sludge into a form of product that is easier and safer to handle in agriculture.
In addition, wastewater sludge contains concentrations of metals where these originate from industrial discharges. It should be possible to reduce their concentration through controls on the industries concerned, in order to remain within the acceptable limits for metals in soils.
2.8.3 Are there any other methods of disposal or beneficial uses of sludge?
In the past, disposal of sludge at sea was commonly practised by urban developments at or near the coast. This practice is prohibited from 1998 by EU Directive, 91/271/EEC.
Considerable work has been carried out on beneficial use options. Particularly noteworthy is the recent research that has been done in Japan. The use of sludge has been considered for fuel, in ceramic production and in the formation of building panels. It has even been used to produce decorative brooches, tie-pins and table mats! At present, these uses are still to be considered experimental and are generally more costly than conventional disposal or agricultural use. They can only be considered by the very largest of wastewater plants serving populations in excess of a million.
Where sufficient landfill space is not available and when the sludge is not suitable for use in agriculture, incineration will have to be considered. Ash resulting from incineration, which will be far smaller in volume compared to the original sludge, must also be properly disposed of according to the same restrictions as sludge.
2.8.4 What happens to screened material as well as grit and sand removed in the preliminary treatment units?
A very important aspect is the removal of sanitary litter e.g. sanitary towel strips, condoms, cotton and sticks. Very unsightly, blocks equipment, major PR problem in sludge use.
Paper, rags, pieces of timber, etc. arriving through the sewers at the wastewater treatment works can block pipes and damage machinery and processes. This material is removed by screens at the inlet to the works. Frequently contaminated with sewage solids, screenings should be separately stored, preferably after a washing process and strained of excess water. They should either be burnt on site in an appropriately designed incinerator, or carted into the sanitary landfill used for the disposal of solid refuse.
Sand and grit enter sewers principally from roads and can damage machinery and accumulate in process pipework. In order to avoid this, and to protect the works, sand and grit, are removed in one of the first treatment units. Sand and grit should also be disposed of to a sanitary landfill. If units removing the sand and grit are operating correctly, the material should be inoffensive to handle but, if not, washing prior to disposal may be necessary.
Fig. 2.7 Grit and sand is disposed of to sanitary landfill