TLTG "Biogas dan Kompos"

  LOW-COST WATER SUPPLY AND SANITATION

“Biogas and Composting”

COMPOSTING

Composting involves the biological degradation of the organic components of wastes which have a relatively high concentration of solids. The product is humus-like in content and is an excellent soil conditioner and partial fertilizer. Initially, psychrophilic and mesophilic bacteria (10-40^oC) present in the organic waste, decompose it and generate heat. The temperature rises until it limits the growth of the mesophilic bacteria. Thermophilic bacteria (40-70^oC) then take over and decompose the waste further. They generate more heat until the temperature and nutrients limit the growth of the thermophilic becteria. The temperature then begins to drop and mesophilic bacteria take over again as decomposition approaches completion.

There are certain factors which have to be in desirable ranges for efficient aerobic composting to occur  at high temperature.

1)    An optimum balance in the carbon (C) to nitrogen (N) content is neccessary because the bacteria need a minimum supply of nutrient to survive. The starting optimum C:N ratio of most wastes is about 25 to 30:1.

2)    A moisture content of about 50 percent is essential for good composting. Moisture levels of above 65 persent cause anaerobic conditions and low temperature. When the moisture content of a composting mass drops much below 50 persent, the compost process will be slow.

3)    The optimum pH range for most bacteria is between 6 and 7,5.

4)    Because composting is an aerobic process, the availability of air is key factor to its success. However, it is very difficult to determine the true oxygen requirement because this depends on so many variables: temperature, moisture content, availability of nutrients, etc. An effective and inexpensive means of monitoring oxygen is to check the compost for foul odors. If these are present, it indicates an insufficient oxygen supply.

5)    Temperature is also a key factor affecting biological activity. The range of optimum temperature for the composting process (45 to 65^oC) is quite broad because of the many groups of organism taking part in the process. The temperature caused by thermophilic bacterial activity result in the destruction of disease-causing organism.

BIOGAS

Biogas

The basis of the biogas production is anaerobic digestion in which waste are decomposed without oxygen at relatively high moisture contents (90 to 99,5%) . The waste undergo decomposition, first producing volatile acids and then biogas from the volatile acids. The biogas  consist mainly of methane (about 65 %). Carbon dioxide (about 35 %), and trace amount of ammonia, hydrogen sulfide, and other gases. The peocess also involves break-down of proteinaceous materials intoamines and such fertilizer as nitrites and ammonia. These products are more easily available as nutrients for plants, than are the complex proteins in the original waste.

Advantages of the Biogas Digester

the biogas process can be a great benefit to rural communities in developing countries for a number of reasons :

-          As appropriate technology the biogas process stresses self-reliance.

-          There is less dependence on improted commercial fuels and therefore there is the potential to save foreign exchange.

-          Biogas is more thermally efficient than fuelsuch as firewood. As an alternative fuel it helps to abate the serious problem of deforestation.

-          Biogas technology also has the potential to recycle nutrients back into the soil.theburningof dung and crops residues leads to an ecological imbalance because the chemical nutrients are lost from the ecosystem. Biogas production from organic materials permits energy production, yet preserves the nutrients, which are recycled back into the land in to form of slurry.

-          The slurry from thr biogas digester is used as a fertilizer. It caries nearly all of the fertilizer value of the influent frrd material, but in a more useful digested form. The nitrogens, for example, are more available as plant nutrients, having been broken down from complex organic forms to ammonia.

Problem and Constraints of the Biogas Digester

The biogas digester does present some difficulties of technical, economic, and social nature which must be worked out before the digester can become widely used.

                                                                                     

Technical Problem

·        Methane generation stops at temperature lower than 10 C and since this make the production of methane dependent on temperature, the operation of digester is limited in colder regions to be warmest months. Unfortunately, surplus gas produced in the summer can not be economically or particallystord for winter.

·        Digester must be regularly fed with proper proportion of dung and water --- an important point to be considered. Farmers often do not pay enough attention to this. Some farmers feed a slurrythat is very dilute, while others feed a sluury with a high solids content. The result is poor gas production. Good planning on the part of the farmer is required to ensure that the correct mixture of slurry is always available.

·        The bigaspalnt must be located close to water because 50 to 100 litersof water are required to produce 2 m³ of biogas per day (enough for cooking purpose for afamily of four to five). Biogas plant may be unfeasible in areas where water is scarce or labor for transport is required.

·        Problems can develop with corrosion of the gas holder in the floating cover design.

·        Generally there is a lack of technical knowledge and capability at the village level. Human resources must be developed at this level to ensure proper construction, maintenance, and operation of the biogas units.

Social Problem

Household digester which use castle  dung required the dung of two to three head of castle every day. If human waste is used, lees castle dung is needed. People may not want to handle human waste, but a demonstration plant and user education will help make this an acceptable practice in a community. Pour-flush toilets discharging to the digester are particularly appropriate because there is need to handle the excreta.

Economic Problem

·        The financial viability of biogas plants depends on whether the gas and slurry they produce can substitute for the fuels, fertilizers, or feed which previously had to be purchased or paid for in kind. If cash or in-kind savings can be used to repay the capital and maintenance costs, the plant will be financially viable. Many farmers, however, have no sleady income and get their money only two or three time a year. They also have other demands on their income. Given this situation, many rural families can not afford a digester. A possible solution is financilal assistance for biogas units in the form of revolving loans.

·        The main constraint on the use of biogas digester are not economic or technical, but are social and organizational. A successful biogas program must incorporate major human resources development and organizational component at the village level.

Types of Digester

There are three main variations of the biogas digester.

Fixed Done Digester (China)

Over 7 million plants have been

In China, the systems that has developed uses a fixed masonry top to collect the gas as shoen in Figure 2. Pihg and human waste are fed daily into the inlet chamber, and drawn out of the effluent  on a regular basis. Gas build up under the dome forcing the liquid up the inlet and outlet chambers. This type of digester can digest plant waste as well as animal and human waste. Iy is ussualy  built below the ground, which makes it easier to insulate in a cold climate. The digester can be built from bricks, concrete, lime or lime clay. It is easier to insukate I a cold climate. The digester can be built from bricks, concrete, lime or lime clay --- thus local materials  and labor can be used. Difficulties are ecountered in ensuring thet the masonry dome has no cracks. This necessitates training masons in their special skills required. The Chinese biogas program now emphasizes rehabilitation of the failed units which are reportedly as many as two million or the seven million. The chinese program Is looking into the floating dome and plastic unit described below.

Floating Gas Holder (India)

The floating dome made of sheet iron is predominant in India. It is based on Khadi Village and industries commission (KVIC) design which collects the gas under a floating dome illustrated in figure 3.

The digester runs on continuous basis and generallyuse castle dung as input material. The gas holder is ussualy made of steel, although ferrocement and bamboo cement have beenintroduced as substitutes.the original version of the floating gas digester was vertical cylinder provide with a partition wall. Problems such as rustling of the dome and the costs of the unit, have limited its use to the land-owning farmer.

The Indian biogas has more recently begun to develop the fixed dome concept in the Janat plants based on the Chinese design.

Flexible Bag Digester *China

Plastic sheeting is being used in China for digesters, PVC with an additive (red mud) for solar radiation protection is formed into a bag or move simply used as a top in the “semi plastic” digester illustrated in figure 4

Potential Gas Production

The optimum condition for biogas production are temperature ranges of 30 to 35 C, a solids content between 7 and 9 %, and retention periods of 25 to 35 days. However, in most cases it is not economically or technically feasible to control the temperature to fit the prescribed ranges. The production of biogas virtually cases at temperatures below 10 C, which is a constraint for cold regions. Most of the waste materials have solids content in excess of 10% in such instance. Water must be added to the waste feed.