Why Gasification of Sewage Sludge is Better Than Spreading it on Land

Sewage sludge actually seems to be too wet to burn, but it does!

Gasification of sewage sludge is becoming a very popular method for disposing of the organic sludges from sewage works, or to give them a more genteel name wastewater treatment plants. It is currently a large market area for gasification technology. So, how did this come about and what are the particular merits of gasification when sludge disposal is considered. In this article we will explain this.

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First let/'s be clear what sewage sludge is. Sewage sludge is the bulk of the residual material removed during the wastewater treatment process. Most wastewater plants have dewatering facilities which compress and squeeze the water out of the collected sludge from the treatment process. In these cases the sewage sludge which goes into a gasification plant is the solid, semisolid, or liquid organic material that results.

You may also hear talk of biosolids. The terms sewage sludge and biosolids are used by US EPA interchangeably, but others often use the term biosolids to describe sewage sludge that has had additional processing for land application.

For gasification to take place the sludge is is usually transformed into biosolids using a number of complex treatments such as digestion, thickening, dewatering, drying, and lime stabilisation.

Sewage sludge can be composted and it can be spread on land. However, it also unfortunately is known to contain a heavy metals, PAH's and other organic micropollutants, and pathogens like spores of Cryptosporidium and Giardia. Apart from substantial direct health risks for humans. This sludge, if applied to land, also results in contamination of the environment due to the presence of ammonia, and BOD etc. It is in fact an amalgamation of most the wastes of our society and it is not closely controlled so it may contain highly undesirable polluting trace contaminants.

Sewage sludge is an inevitable and unavoidable by-product of sewage treatment. The amount produced is massive and is also expected to rise by rapidly in Europe in particular, mainly as a result of the higher treatment standards provided through the UWWTD.

To achieve gasification sewage sludge is placed into a dewatering or dehydration chamber. The low water content sludge cake remaining is heated to convert moisture in the sludge to steam and cause gasification. These facilities are usually large because sewage sludge is produced in huge quantities day after day, year after year. In fact the municipalities find themselves under relentless pressure to get rid of the stuff.

Sewage sludge is primarily made of water, and 90 to 95 percent of its mass can be eliminated through the thermal drying process. The cost of thermal drying is usually easily repaid as the remaining 5 to 10 percent is much less expensive to transport and burn than the original material leaving wastwater treatment tanks.

Sewage sludge has been historically in the UK and US spread on land or shipped out to sea and dumped. However, recently it has been realised that it is not suitable to apply on areas near ponds, lakes, rivers, and streams without appropriate buffer areas (zones) and it is not suitable to apply near wetlands and marshes without a permit (for example this is now regulated by US EPA, 1995). Dumping at sea has also been outlawed by many nations, including the EU.

Furthermore, steep areas with sharp relief and undesirable geology and soil conditions are not suitable areas to apply sewage sludge (US EPA, 1995). Sewage sludge is classified in different countries in different ways but for most it falls into three categories according to application to land: unrestricted, restricted and unsuitable.

In many nations, sludges classified for unrestricted use may be applied in an unrestricted manner to all lands excluding sensitive sites, while those deemed unsuitable cannot be used outside the boundaries of the source sewage treatment plant. However, the rules are being tightened and it is becoming more and more difficult to spread sludges on land.

One major problem which is becoming much more appreciated is the long-term build-up of contaminants such as heavy metals in the soils where sludges are spread. Once these metals build-up beyond certain levels the land would become unsuitable for growing crops due to the presence of these metals in food grown on that land. Clearly, loss of productive land in this way has to be avoided.

Commentators have also identified fear of liability as a major deterrent to the widespread land application of sewage sludge. Liability issues regarding land application include not only legal liability, but also a risk to food product marketablity as a result of negative public perceptions of the land application of sewage sludge.

So now, many municipalities responsible for sludge disposal have turned to the benefits of gasification, and a small but well experienced industry is starting to form to use that sludge. These new companies are developing gasification technoogy to generate heat and power, and from gasification will also come useful gas (syngas) which can be used as a raw material for the production of many chemicals which have until now only been produced from non-renewable carbonaceous sources.

Steve Evans is a renewable energy enthusiast. Far from being despondent about rising gas prices he sees it as a great opportunity for us to start using clean renewable gasification energy sources which are going largely to waste all around us.