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.

Presentation on videoof plasma gasification

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.

Gasification of Biomas from Wood Pellets to Algae

Wood pellets - when compressed these hold together well - but concern arises when they become wet

Gasification of biomass, which is a renewable and sustainable energy source, is a promising alternative to using fossil fuel based fuels, at high and extremely variable cost. Energy sources will come from an increasing list of biomass resources - sources of energy which have gone largely ignored and unused for more than 50 years. The list is likely to be so wide that even algae are likely to be produced to meet the demand for cheap and sustainable green energy in the future.

LIVE EVENT David James - Gasification of Algae - Green Fuels. Watch it now!

Combining the application of biomass gasification - where a mixture of mainly hydrogen and carbon monoxide is generated - with fuel cells - where the hydrogen is electrochemically converted into electricity - offers a highly efficient and incredibly flexible energy source. Gasification systems basically consists of a gasifier unit, purification system and energy converters - burner or engine.

Wood Pellet Gasification

In one not atypical gasification system fuel, initially wood pellets, is supplied from an indoor wood pellet storage site into the gasification plant. The fuel is fed through lock hopper systems by feeding screws into the lower section of the gasifier’s fluidized bed.

Fuels which can be used without difficulty include ground seed corn, switchgrass, cornstover, RDF (refused derived waste - subject to licensing constraints), woodchips, and sawdust to name just a few. The gasifier is a fluidized bed gasifier meaning that a tube of sand (the bed) is mixed (fluidized) by passing air through holes in the bottom plate of the tube.

Producer gas is created in the gasifier and this isa  fuel gas and chemcial source which can be used to replace fossil fuels in a wide range of devices, including road and rail transport.

Other devices which could be retrofitted with gasifiers are furnaces for melting non-ferrous metals and for heat treatment, tea dryers, ceramic kilns, boilers for process steam and thermal fluid heaters. Producer gas can also commonly be used for commercial cooking applications.

Gasification of wood and wood-type residues and waste in fixed bed or fluidised bed gasifiers with subsequent burning of the gas for heat production is state of the art in Northern Europe. However, the market has hardly yet been expoited aven in those countries where the wood gasifiers which are employed in the Scandinavian countries are used almost almost entirely just for heat generation.

Biomass is a sustainable energy resource which is destined to play an important role in future energy production and as an organic chemical raw material supply source, with biomass gasification the most promising conversion technology and very effective in reducing greenhouse gas emissions.

Biomass is converted into a gaseous mixture of hydrogen, carbon monoxide, carbon dioxide, and other compounds by applying heat under pressure in the presence of steam and a controlled amount of oxygen (in a unit called a gasifier). The biomass is broken apart chemically by the gasifier's heat, steam, and oxygen, setting into motion chemical reactions that produce a synthesis gas, or "syngas"-a mixture of primarily hydrogen, carbon monoxide, and carbon dioxide. It is this synthesis gas which can then be further reacted and refined to replace oil as a raw material for use in refineries.

Finally, we said that we would be telling you about the, improbable as it may seem, use of algae as a biomass energy source.

Algae Energy Farms

Algae energy farms which might be light enough to be integrated into large, flat rooftops and could therefore be present even right in the centre of our cities could be manufacturing green energy from the sun on a daily basis. If factory and commercial buildings could support the carbon recycling systems on-site, the waste heat and other byproducts of the carbonizer and fuel cell could be used for industrial process heat or space heat. In addition algae has been a hot topic lately as a possible solution to most of our fuel and food needs. Nearly half of an alga's mass is made up of lipids (a major component of vegetable oil) that can be used in the production of biofuels, and all sorts of plastic and the chemicals now made from oil.

So, if someone you know suggests that you should consider using gasification in some way, don't just laugh and dismiss him as a crank. He just could be right!

Steve Evans is an anaerobic digestion and methane energy enthusiast. Far from being despondent about rising gas prices he sees it as a great opportunity for us to start using clean renewable wind turbine energy which is usually to be found all around us. Also visit the < a href="http://www.dog-breeds.me.uk">dog breeds web site.