Domestic Waste and Biogas Technologies

E.S. Pantskhava, Close JSC Center «Ecoros», Moscow

Recently, many callers to the editorial office have requested to tell about an Israeli domestic waste processing technology ArrowBio. Having familiarized ourselves with the materials sent to us, we were surprised to know that the waste two-stage methane fermentation technology used in ArrowBio was developed in the USSR as early as in 1961 and was implemented in industry. Although this technology is not widely used nowadays (due to various reasons), the editorial team found it necessary to tell our readers about its peculiarities, bearing in mind that almost everything old is new again.

BIOMASS – THE SOURCE OF ENERGY

A little more than 35 years have passed after the time of the world energy crisis, which was a powerful stimulus for solving of three problems:

• conservation of all sources of fuel and energy existing in the Earth;
• more efficient use of all biomass kinds as sources of permanently renewable energy;
• maintaining Ñ0₂ balance in the Earth atmosphere and decreasing the "greenhouse effect".

The last two problems are closely interconnected.

As a result of the processes of photosynthesis, the most powerful converter of the solar energy via conversion of Ñ0₂, the globe annually produces over 200 billion tons of biomass (dry weight); i.e. the carbon content in biomass being about 50%, up to 300 billion tons of Ñ0₂ are appropriated on average during photosynthesis. For this purpose should be spent 3-10²⁴ J of solar energy.

Almost the same amount of Ñ0₂ is emitted into the environment as a result of natural dying and decomposition of the organic mass, and just a small portion of transformed organic substances leaves the natural carbon circulation, accumulating at the bottom of bogs (peat) and lakes (sapropel), or is carried by ground and underground waters to the deeper strata. This balance of Ñ0₂ should be maintained in order not to increase the "greenhouse effect".

One of the ways to decrease this greenhouse effect is to reduce the use of traditional energy sources through wider use of nontraditional energy carriers, including biomass.

Most part of the accumulated biomass gradually transforms (mainly due to the complex food-chain of the biosphere) and finally oxidizes to Ñ0₂. According to the energy conservation laws, this decomposition and oxidizing is accompanied by output of energy, which is emitted into the outer space. Only in the territory of Russia the natural processes result in annual dispersion into the space of an amount of released energy equivalent to several billion tons of oil. Is there a possibility to accumulate a part of this emitted energy for use as commercial energy, without disturbing the global natural processes and Ñ0₂ balance? Modern science and engineering say such a possibility is real.

In Russia is annually formed over 229 million tons (dry substance) of organic waste of plant and animal origin. Particularly, in agriculture they amount to 209 million tons (animal and poultry farming - 59 million tons, plant growing - 150 million tons). In cities, the amount of solid domestic waste (SDW) does not exceed 20 million tons. The energy content of such an amount of waste is equivalent to 81 million tons of reference fuel.

The nature of the biological process of organic substance decomposition with generation of methane has not changed over the past millenniums. But the modern science and engineering have created equipment and systems allowing making these "ancient" technologies profitable and applicable not only in the warm climate countries, but also in the countries with severe continental climate, like Russia.

SDW PROCESSING TECHNOLOGIES

The following SDW processing and sterilization technologies are known:

• burial at landfills for natural waste decomposition for many decades. The SDW half-life in such conditions is 30-60 years;

• waste incineration and gasification;

• processing of commercially profitable fractions: PET, metal, glass etc;

• biothermal treatment (active composting) – microaerophilic process with generation of heat energy and solid organic fertilizers;

• biogasification: passive – in ground carts, active – in stationary bioreactors-methanetanks;

• incineration in gas generators with generation of combustible gases (heat and electric energy) and slime, which is then used as construction material (pavement).

The above technologies have certain drawbacks preventing their wide implementation:

• SDW incineration virtually destroys the valuable organic component, being the feed stock for obtaining organic fertilizers and pollution-free fuel (biogas), which is especially important for small towns and large settlements that do not have industrial enterprises and hence their SDW are not contaminated with heavy metals;

• waste incineration causes gas emissions into the atmosphere;

• SDW separation on the one hand fails to solve the problem of complete sterilization of the SDW organic component used for further composting, and on the other hand this process (composting) transforms into Ñ0₂ up to 50% of processed carbon (potential source of heat);

• biogasification in piles is slow and lasts for several years, hence it does not solve the problem of land alienation, although it becomes smaller than when arranging landfills;

• 90% of the remaining SDW are piled in waste dumps, requiring continuous alienation of new lands around cities (usually cultivated), since the half-life of the SDW is 30-60 years.

ARROWBIO TECHNOLOGY

Today's ecological problems require solving quite a concrete task: creation of stationary, continuous, wasteless and highly profitable plants with a short organics processing cycle (no more than 25-30 days) and application of the SDW organic component processing technology that allows:

• industrial processing of SDW without occupying land;

• reducing the processing to several weeks against several years;

• ensuring complete sterilization in accordance with the current sanitary norms;

• creating profitable process with obtaining of commercial products: pollution-free highly efficient organic fertilizers and gaseous fuel.

The development of such a technology is a global problem. Several countries have been dealing with it, including Israel, where ArrowBio technology was patented, allowing production of biogas from domestic waste, which gas may be used in power plants. This technology was tested for 5 years in laboratories and field conditions at a semi-industrial plant near Khadera (Israel) and was later applied at a plant in Tel Aviv. This technology was approved by specialists from Israel, USA and other countries as a more efficient and saving SDW processing method compared to conventional ones.

During the domestic waste processing, the ArrowBio technology allows preventing such dangerous effects as emission of methane and carbon dioxide into the atmosphere (inherent in waste decomposition at landfills) and greater "greenhouse effect" caused by these gases. 

Figure 1 – ArrowBio process flowsheet

The main advantages of the offered technology are as follows:

• formation of biogas used at electric power plants and municipal transport sector (with far lesser amount of harmful emissions);

• over 90% of sorted out domestic waste materials (metals, plastics, glass etc.) return to the economic circulation;

• reduced "global warming" effect;

• atmosphere, soil and water pollution is eliminated;

• production of high-quality compost (fertilizer).

Solid domestic waste is crushed and then, in the presence of effluent (eluate), hydro-crushing is performed with simultaneous extraction of polymeric, ferrous and non-ferrous waste, glass waste and wood waste.

The cleaned organic mass is subjected to biohydrolysis and acidic anaerobic fermentation with formation of volatile fatty acids and lower alcohols. Acidic brew goes to the second bioreactor, where mesophilic methane-generation takes place. The methane brew is separated into liquid effluent, which goes to the second tank with hydro-crushing, and solid sediment, which is used as biological fertilizer (fig. 1).

The ArrowBio technology implements two-stage methane fermentation: acidogenous and methanogenous stages, which allows reducing the processing time.

There is no doubt that such a technology is interesting, but its acquisition requires greater specification of the basic flowsheet parameters and great capital expenditures.

RUSSIAN EXPRESS-TECHNOLOGY FOR SDW PROCESSING

The two-stage methane fermentation was first developed in the USSR in 1961 and was used in industry for processing of acetone-butyl dreg by a method of thermophilic methane fermentation in production of vitamin Â₁₂ and biogas. In the 1970s, the two-stage process for SDW processing was implemented in Sweden.

In the late 1980s, the A.N. Bach's Institute of Biochemistry under the Academy of Sciences of the USSR developed a technology of recirculation-hardphase thermophilic methane-generation of the SDW organic component into biogas and fertilizers. The fundamentals of this theory made it possible to create an express-technology for processing organic fraction of SDW into gaseous fuel (biogas) and organic fertilizers; processing time - 2-3 weeks instead of 30-60 years at landfills.

The technological system included two bioreactors-methanetanks: a reactor for hardphase methane-generation (fermenting mass humidity 75-80%) and a reactor for liquid-phase methane-generation (humidity 97-98%). The liquid phase (using pumps) was continuously pumped through the hardphase reactor, which ensured fast decomposition of organic substances in thermophilic stable conditions. The results of one of such pilot experiments are shown in table 1, 2.

The volumes of the pilot methanetanks - 110 liters each, fermentation temperature - 55°Ñ, SDW samples were taken from dustbins in one of the residential areas of Moscow. Metal and glass were not removed. Large paper pieces were pre-crushed.

For a more convincing evidence of the fermentation process stability when using this technology, the organic component was complemented by easily decomposable organic food waste, including crushed milk product boxes.

The number and composition of the gases, as well as other process parameters, were analyzed using the well-known methods. Liquid phase recirculation was preformed with such a speed that the total working volume in the first reactor (SDW + inoculums) remained unchanged.

The obtained data showed that in the process of the thermophilic recirculation-hardphase processing of 2.5 kg of SDW, in 33 days were generated 584 liters of biogas containing up to 68% of methane; also, in the second reactor, the average methane content in the process of fermentation was 85.5%, and the ðÍ value, one of the main parameters reflecting the process conditions, was maintained at the optimal for this process level of 7.25-8.25. At the same time, in the control variant, where the process took place by one of the classical methane-generation methods, it stopped on the fifth day due to accumulation of acid products that inhibited development of methanogenous bacteria, which was confirmed by lowering of the medium's pH down to 4.37, and the final pH vale was 4.65. The biogas output did not exceed 44.6 liters or 4.8 % of the biogas amount formed in the experiment variant. Methane content – 10.3, Ñ0₂ – 76.5, hydrogen -13.2 %.

All this evidenced domination of acidogenous stage of the process in the control variant, where organic substance conversion was only 16%. The biogas output per 1 kg of processed SDW was 374 liters, methane - 253 liters; per 1 ton, respectively: 374 and 253 m³. The dry substance output of organic fertilizers (slime) was as much as 0.41 kg or 16.4 %, and 2.1 kg at 80% humidity.

Over three weeks of fermentation (21 days), the biogas output was 882 liters, or 94% of the total volume of the generated biogas; methane - 585 liters or 92.6%. The degree of decomposition of the SDW organic substances, assessed based on the formed biogas, over 21 days, was 68%, while over 33 days it was 71%, i.e. SDW processing by the suggested technology may be performed for 15-20 days. Thus, the recirculation-hardphase thermophilic processing of SDW creates stable conditions for maintaining optimal values of some physical-chemical and biological parameters, which ensures fast SDW organic content decomposition process with formation of considerable amount of biogas with high methane content. Inclusions of other nature (metals, glass, rubber etc.) do not inhibit this process. Based on the conducted researches, a flowsheet of recirculation-hardphase thermophilic methane-generation of the SDW was suggested, presented in fig. 2. The SDW processing time under this technology is 10 days. The waste is crushed and then fed to the hardphase fermentation bioreactor-methanetank by an auger conveyor.

Figure 2 – Diagram of express-technology for SDW processing using recirculation-hardphase thermophilic bio-methane-generation method

After loading the SDW, the first reactor is inoculated, i.e. filled with a liquid fraction containing methanogenous biocenosis, and the total substrate humidity is brought to 80%, but not more.

Then, the substrate mass is heated up to 52-53°Ñ. This temperature is maintained automatically by heat casings located around the reactor or by internal heat exchangers. Any options are possible subject to just two conditions: profitability and practical feasibility of the process. In one day after the inoculation begins the process of recirculation (either periodically every 4-5 hours, or continuously) of the liquid fraction from the second bioreactor (liquid-phase methane-generation).

The low-alkali methane brew containing active bacterial mass self-flows to the upper part of the first reactor, where an active zone is created, in which easily decomposable organic substances are converted into acid products, partially neutralized by the methane brew and removed by the same from the active zone to the lower inactive part of the reactor. From there, these acid products are pumped to the second reactor.

Recirculation constantly maintains in the first reactor active zone:

• high density of bacteria;

• neutral pH value optimal for active development of methanogenous biocenosis and stable process behavior.

The plant using this technology may be connected with aeration stations for treatment of urban and municipal waste water, where the active methanetanks will act as liquid-phase reactors.

The hardphase bioreactors-methanetanks may have vertical or horizontal design. Their height will depend on the filtering property of the processed mass itself, which in turn will depend on the density of the latter and the pressure it creates. The plant using this technology in combination with plants-separators will allow creating integrated, pollution-free, practically waste-free and profitable enterprises for SDW processing in any climatic region, any city and settlement, with output of ferrous and non-ferrous metals, construction materials, gaseous fuel, electric and heat energy, and fertilizers, without alienation of useful lands.

This technology may receive the widest acceptance in small towns or settlements, where industrial waste, usually containing amounts of heavy metals exceeding the maximum acceptable limits, is reduced to a minimum, and the slime received after processing may be used as organic fertilizer, which considerably increases profitability of such plants.

A town of 20 thousand dwellers produces 14-17 t/day or 5-6 thousand t/year of SDW. Its processing may give 1.75-2.1 million m³ of biogas per year and up to 4-4.8 thousand t/year of organic fertilizers at an 80% humidity.

Practical experience of using biogas technologies in various regions of Russia has shown that it is not only one of the ways for obtaining technical fuel from biomass, but also a real way to increasing the growth of the biomass itself through the use of the fertilizers obtained with these technologies, containing natural high-activity growth stimulants of auxin class, which in turn facilitate additional utilization of excessive carbonic acids in the atmosphere.

Solid domestic waste, this permanent scourge of all cities in the world, through the latest biogas technologies may not only be fully and quickly processed without damage to the environment and agriculture, but also become good raw material for obtaining construction materials, fuel, energy and fertilizers. Russia will not be an exception (among the market economy countries) as to the use of biomass (various organic waste of agriculture and SDW), which may be used for obtaining fuel and energy, especially in locations distant from the centralized energy supply, as well as for obtaining high-quality organic fertilizers having great potential for better growth of the initial biomass.

BIBLIOGRAPHY

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