|GATE - 1/95 - Waste Water: Resource Management and Environmental Hygiene (GTZ GATE, 1995, 56 p.)|
Recent trends of anaerobic technology for the disposal of solid and liquid waste by Hartlieb Euler and Andreas Krieg
For many years the rationale behind using biogas technology (or anaerobic technology) in developing countries was the search for renewable sources of energy. Nowadays environmental protection aspects are the significant factor. A technology which previously just filled a "niche" is now becoming a key technology for integrated, solid and liquid waste-treatments concepts both in industrialised and developing countries.
The dominating motive behind using biagas technology was to produce energy and fertiliser on a decentralised basis for small farms. The initial dissemination programmes assisted by German development cooperation therefore centred on adapting the technology on site for agricultural biogas plants.
Environmental and disposal aspects were only exceptional priorities. Projects for domestic and industrial wastewater treatment rarely applied the anaerobic option.
Over the course of the years additional side effects of farm-based facilities have become significant advantages of anaerobic technology in a rural environment: improved hygienic situation; reduced work load for women and children who are traditionally responsible for fetching firewood; smoke-free kitchens reducing respiratory and eye diseases; orderly disposal of human faeces via latrines in households, community buildings and institutions, lowering the incidence of worm and parasitic diseases which are a common cause of death in many countries.
Experiences from German development cooperation have shown, however, that small farms do not possess sufficient capital to invest in biogas plants to replace firewood without bringing any increase in farm income. Since the mid-1980s, therefore, successful dissemination concepts for small scale plants have been coupled with loan programmes and/or subsidy programmes (see "Lessons from the Thai-German Biogas Programme", gate 3/94).
Planning, construction, maintenance and repair work nowadays is done by specialised, trained private contractors who guarantee the longterm success of the technology
Since the 1980s, constructing biogas plants is no longer a leisure time activity for amateurs. Quality certificates, training certificates and operation licences, detailed building plans and long-term guarantees by experienced engineers and private sector companies have upgraded the image of anaerobic technology even in the small-scale agricultural sector.
Biogas plants for agroindustries
The original priority goal of biogas technology - to improve the living conditions of small farmers - has been replaced by a more pragmatic orientation to environmental and infrastructure-policy goals.
At the same time, integrated rural programs now combine composting, water supply, other renewable energies, agricultural production and processing with anaerobic technology, into a mix which fits the specific location.
For the past five years biogas plants are also becoming a regular feature on larger farms (with 50 to 5000 heads of cattle equivalents) and agroindustries (of all sizes) in industrialised countries, particularly in the food processing industry.
In Europe this trend has been largely influenced by environmental legislation, in many other countries by protests and actions by people living in the neighbourhood of these operations.
Higher electricity prices for feeding electricity from highly efficient co-generation plants into the public grid have become attractive economic incentives for these industries. In the meantime, high returns on investment have become the decisive factor for investment in many countries. The technical reliability and productivity of the anaerobic plant can be guaranteed now.
The technology and marketing of agroindustrial plants and large-scale biogas plants has greatly changed worldwide within the last few years. At the end of the 70ies traditional manufacturers of industrial machinery had only limited success in trying to introduce the technology. The market rarely operated up to anything beyond highly subsidised or often questionable prototypes.
In the meantime, an agriculture based plant technology, often derived from silos or gully container technology, combined with efficient power and heat aggregates have conquered the markets. The main building elements are now made in series production. In particular technical accessories such as gully pumps, mixers, heating and insulation, and equipment for sulphur purification and power-and-heat combination (i. e. co-generation) are almost completely standardised, allowing modular construction with prefabricated elements. This is essential for the success of anaerobic technology which can be adapted individually to the needs of each farm, region or country, using standardised modules which are far more advantageous from both the cost and technical viewpoints.
In Germany in particular, anaerobic technology has opened up new prospects for disposing of agricultural and domestic waste thanks to the co-fermentation system. Co-fermentation means that agricultural waste substances, gully and solid manure is fermented together with organic waste and wastewater from private households (garden waste and grass from lawns, market waste, sewage) and with liquid and solid waste from abattoirs, the beverage and vegetable preserving industry, breweries, dairies, sugar starch and alcohol production, kitchens and canteens, etc.
The successes achieved are founded both on the disposal fees which the agroindustry and local authorities have to pay to the operator for dumping organic solid waste or treating organically polluted liquid waste. On the other hand, the operators of the plant can use the biogas production to generate electricity which is then fed into the public grid.
Because of the different sources of the material being fermented and their possible pollutant contents, some ecological misgivings can be raised against this disposal method. To avoid soil contamination, the substrate must first undergo a detailed analysis before it is used as fertiliser. Furthermore, the transport of materials to be fermented should not require more energy than that which will be generated by fermenting it.
The co-fermentation system reactivates links between agricultural and domestic disposal of liquid and solid waste. Indeed, the anaerobic technology for agriculture was developed in the 1940s in Europe at local authority wastewater treatment level. If only for cost reasons local authorities and agroindustries often copy parts of the processes that agriculture develops and uses to treat solid and liquid waste.
On the other hand, factory farming operations which produce large quantities of organically polluted wastewater each day benefit from using the efficient technology systems operated in municipal wastewater treatment: The "Upflow-Sludge-Blanket" known as USB is one example, now broadly applied. This system feeds the wastewater to be treated into the bottom of the reactor and when flowing up through a bacterial zone the organic parts are rapidly fermented. The purified water can than be discharged into lagoons or rivers .
Municipal water treatment technology
Shortage of public funds has impacted on municipal sewage treatment plants worldwide which traditionally use high-cost and energy intensive (aerobic) water treatment technologies. Centralised wastewater collection and treatment solutions with large-scale sewage networks and centralised sewage treatment plants can only be an economically viable option when a sufficient volume of wastewater is collected.
Numerous promising examples of small and medium sized anaerobic purification plants for domestic, institutional and village waste are now on the market. Smaller urban centres with 5,000 to 100,000 inhabitants however still require more effective, locally-appropriate technology standards - also for anaerobic purification systems which are economically viable and guarantee long-term operating safety.
Municipal solid waste
In warmer climates in particular, centralised sewage systems can cause considerable hygienic problems.
The local ecological water balance also can benefit from decentralised nature based approaches in rural areas. More than 50 % of sewage treatment plants for human excrements in industrialised countries are nowadays equipped with anaerobic purification stages for sludge treatment.
In developing countries anaerobic wastewater treatment processes are installed because they require less equipment and less costs - and consequently save foreign exchange.
Problems arising from the lack of skills and training are met on the management side, however, particularly in remote areas.
In industrialised countries with overflowing waste dumps and emission problems, anaerobic technology is frequently used to supplement or replace aerobic solutions.
This applies both to dumps with mixed waste which collect, purify and use the small parts of the gas obtained, and also to centralised composting concepts for organic waste.
Many towns collect organic waste in so-called "bio bins" from households, markets, gardens and start to treat it in anaerobic reactors as well. The process products are gas and water which is recirculated and often retreated, and a stabilised and valuable residue which can also be used as fertiliser or for soil structuring.
Modern lower-cost fermentation plants for solid communal wastes are basically advanced models of the an aerobic plants used in agriculture. They require only limited industrial processing and control technology. Depending on the amount of disposal fees involved, anaerobic and combined aerobic/anaerobic solid-waste treatment plants are considered a fundable and quite viable investment particularly in European countries - the bioComp system of the T.B.W. Company in Frankfurt is one example (see photo).
Municipal waste treatment using the biocomp procedure developed by T. B. W. is based on combined anaerobic/aerobic composting. 13,000 tons of organic municipal waste are processed in this plant each year. The course material is forwarded to the (aerobic) composting plant The fine material is mixed with the liquid separated from the digested sludge and forwarded to the first fermentation stage. It takes approx. two weeks for the slurry to pass from the top to the bottom of the first-stage fermentation reactor. The active sludge then proceeds to the second reactor via bottom drain. When it passed out of this reactor at least 60 percent of the substrate's original organic content has bean converted into biogas. The digested sludge is press-dewatered and sent for composting. Two thirds of the press liquid returns to the process. The rest undergoes partial mechanical-biological purification. Figure: Awater/T.B.W.
Biocomp - combined anaerobic/aerobic composting
The interested public, waste experts and also political decision-makers in industrialised and developing countries all have limited knowledge of the potentials of anaerobic technology for waste treatment and how it can be combined into integrated environmentally friendly solutions.
Inadequate legislation, lacking control capacities and the arguments of industry against what they call excessively high investments in environmental protection are further constraints.
The high-energy and often high-cost methods traditionally used especially in warmer climates are often preferred by disposal plant builders from industrialised countries because they are more experienced in this area. Considerable efforts are needed in order to obtain inter-ministerial and inter-sectoral approaches in the future.
The anaerobic process is still often considered as a second choice for the treatment of liquid and solid waste. For economic and ecological reasons, however, it will only be a question of time before it is a must to select the economically and ecologically optimal technology mix.
The growing interest by developing countries, by sectoral departments in technical cooperation organisations and at national and international conferences proves this trend.
· The initial costs when introducing the process are still the chief investment constraint to anaerobic wastewater and solid waste treatment in agroindustry. Unless legislative standards are established on the basis of the "polluter pays" principle and unless public pressure increases in particular the industry will continue to delay its treatment effort.
· To treat human faeces it is important to disseminate those standardised systems which are easy to maintain.
Personnel training and sensitisation of the population are equally important.
· Awareness must change. «Waste is rubbish" must be replaced by "Waste is a valuable material". Savings in raw material resources will then take on an economic value.
· Operator and cooperation models of financing are in sight: Plants are prefinanced from the expected operational benefits to the companies, the energy saved and produced and other process products which are being retrieved.
La technologie du biogaz pourrait devenir une technologie clans les strates d'assainissement intCeci s'applique de la m mani aux pays industriels et aux pays en dloppement. L'auteur prnte des exemples et applications provenant de l'agro-industrie et des communes. Les possibilitoffertes par la technologie du biogaz sont toutefois encore trop peu connues a fois des ingeurs et des politiciens.
La produccie bogas podria convertirse en una technologia
clave para las concepciones integradas de eliminacie aguas residuales, tanto
en los paises industrializados como en el Tercer Mundo. El autor presenta
ejemplos de aplicacin la agroindustria y las comunas. Sin embargo, existe un
dcit de informaciobre el biogas, tanto entre los ingenieros como entre