Anaerobic Digestion of Wastewater Treatment Sludge
Anaerobic Digestion of the sludge which is generated in huge quantities during the treatment of sewage is now the for treating and disposing of wastewater treatment sludge. This is a major change which has occurred over the last 15 years. Prior to that, when the energy costs were low, was incineration. However, incineration is now recognized as being not only too expensive, due to current high costs of energy, but also to damaging, due to the very high carbon dioxide emissions incineration incurs.
Looking back in time, when modern sewage treatment began in the UK, the major cities all loaded their sewage sludge into barges or specially equipped seagoing vessels and simply dumped the sludge out at sea. That was the lowest cost option and still would be if international marine treaties had not banned the marine discharge of sewage sludge. The the next step in sewage sludge disposal, adopted by the majority of the sewage undertakers was to spread the on land, as a fertiliser.
However, due to the presence of heavy metals in domestic sewage which remained within the sludge, and tended to stay in the soil and build-up toward levels that would eventually mean that crops grown with themselves contain these metals at concentrations that would make them dangerous to eat.
Although, the spreading of sewage sludge on land is still practiced in rural areas, where metals content in sewage is anyway much lower than in industrial areas, it soon became apparent that an alternative was necessary. That alternative was incineration, but as anyone who thinks deeply about this will realise, incineration makes very little sense when the alternative of anaerobic digestion is considered.
To put it simply, in AD process takes sewage sludge (a form energy), and releases that energy for productive use, whereas incineration takes this form of energy and burns yet more energy, just to destroy it and send it up a chimney where it becomes just one more carbon emission increasing climate change even more.
In comparison with other locations of the AD process, digestion and of sewage sludge is greatly simplified due to the fact that sludge is produced by the same organisation and by a known treatment process (i.e. the main sewage works treatment facility). That means that it is much easier to control the quality of the feed sludge, and is the case for a general AD plant taking waste under contract from a variety of sources.
However, what does make sewage sludge digestion more difficult, is the fact that more directly reactive organic matter will have been removed during the anaerobic sewage treatment stage, through which the sludge settles out. Unlike anaerobic digestion the organic fraction of municipal solid waste, and wastes like household food waste, which release in their organic content comparatively easily, sewage sludge needs pre-treatment before it can be successfully digested for a good gas yield per cubic metre.
For that reason, all sewage sludge AD plants have an added stage before the sludge enters the digester tank. The purpose of that stage is to break down the particles so that they release their energy. The most popular pre-treatment stage is known as hydrolysis. Hydrolysis simply means heating for sludge to a temperature above normal boiling point, under high pressure, and maintaining that temperature and pressure for a set period of time.
There are alternatives to using hydrolysis to do this, microwave ultrasound has been used, and the addition of chemicals can have the same result. However, there is a biogas plant is already producing its own heat and power and seems to be little reason for not using some of that heat and power in the hydrolysis process. In fact the hydrolysis process, we will implemented, should release much more energy from the sludge than the sacrificial energy required to run it.
This must not forget that the primary reason for carrying out anaerobic digestion on sewage sludge is also to treat the sludge so that it can be disposed of in a sanitary and loan pollution manner. Thankfully, the digestate from sewage sludge anaerobic digestion plants meets that requirement very well.
There is still of course a need to ensure that heavy metals are not discharge to land when the digestate is used as a soil improver and a fertiliser. Better control of industrial discharges has greatly reduced the metals content of sewage sludge, and in all but the most heavily industrialised areas of problem may no longer exist. However, much of the metal content may be found in the fibrous digestate and if so, after a period of composting in the open air coupled with dry, this material may be incinerated or incorporated into such products has building insulation board, or other non food related uses.