The following points highlight the top three methods used for the treatment of solid wastes. The methods are: 1. Landfill 2. Composting 3. Vermicomposting.
Method # 1. Landfill:
This approach is used to treat solid wastes, like garbage, and the solids remaining after waste treatment. The wastes are used for landfill in which a natural or man-made pit or hollow is filled with the waste, covered with soil and often landscaped.
The site of landfill is carefully selected to avoid subsequent problems; it is highly desirable that landfills are located in an unused area or a derelict land. The waste is collected and may be pre-treated in some way before being placed in the pit.
The pre-treatment may be:
(i) Sorting of the wastes,
(ii) Mechanical pulverization or even
The landfill practice may be divided into two classes on the basis of the type of pit used:
(i) Cell emplacement and
(ii) Trench method or ‘cut and fill’ method.
(i) Cell Emplacement Method:
In this method 2.5 m deep cells of suitable size are excavated at the site of landfill, their size depending on the amount of garbage to be dumped each day. Everyday the waste dumped in the cell is compacted and covered with about 20 cm deep layer of soil. The cells may be designed to be single or multi-layered.
A single layered cell is filled to the top by the solid waste of a single day and covered with soil. In contrast, multi-layered cells accommodate the waste of two or more days; each day, the waste is compacted and covered with 30-40 cm soil, and when the cell is full it is covered with 60-90 cm soil layer.
(ii) Trench Method:
In this method, long trenches are dug, filled with waste and covered with soil. In both cell emplacement and trench methods the soil for covering the waste is dug from the site of the next trench/cell.
However, since about only 20% of the cell/trench volume is occupied by soil, 80% of the soil excavated from the second and subsequent cells/trenches will have to be moved away, unless the level of landfill site is to be raised above the ground level.
Uses of Landfill Sites:
Landfill sites can be useful in the following two ways:
(i) As a source of biogas, and
(ii) For reclamation of derelict sites to develop landscaped gardens etc.
i. Landfill sites generate considerable amounts of methane, which leaks from the soil cover. This presents a fire hazard and gives foul odour, but it can be collected and used as biogas.
ii. The landfill site may be landscaped and planted with vegetation. But problems may arise due to toxic substances present in the waste or produced due to degradation of the wastes, and many plant species may not survive.
One way to surmount this difficulty may be to seal the surface of landfill and put another layer of topsoil. But in such cases, the methane must be lapped and collected to be burnt away from the site or used as biogas.
Hazards of Landfill:
The disposal of wastes in landfills presents several hazards including:
(i) Fires in the waste materials,
(ii) Increase in the population of disease vectors like flies,
(iii) Offensive odours,
(iv) Methane leakage, and
(v) Leaching of toxic and corrosive materials into surface and underground waters.
The water (leachate) from landfills may contain 6000-7000 mg/l total solids, COD value of 1000-2000 and 800 mg/l sodium, while the outflow from landfills may have 1000-2000 mg/l total solids, COD value of 70-80 and 300 mg/l sodium. The risks of Fires, offensive odours and increased vector populations may be circumvented by covering the waste with soil on daily basis.
The risk due to methane can be removed by burning or tapping. Similarly, the damage to the environment from landfill leachate may be avoided by lining the pit with an impermeable material like clay, soil-cement mixtures, concrete, polymeric materials and asphalts. But long- term containment with some of the linings, e.g., clay, is questionable. Therefore, specific pre-treatments of the wastes to reduce toxicity is preferable.
Method # 2. Composting:
Composting is a self-heating, substrate-dense, managed microbial system, and one solid-phase biological treatment technology. This technology is suitable to the treatment of large amount of contaminated solid materials. However, many hazardous compounds prove resistant to microbial degradation due to their complex chemical structure, toxicity and compound concentration that hardly support growth.
Microbial growth is also affected by moisture, pH, inorganic nutrients and particle size. Since composting of hazardous wastes typically involves the bioremediation of contaminated substrate-sparse soils, support of microbial self-heating needs incorporation of proper amount of supplements.
The hazardous compounds reported to disappear through composting includes aliphatic and aromatic hydrocarbons and certain halogenated compounds. The possible routes leading to disappearance of hazardous compounds include volatilization, assimilation, adsorption, polymerization and leaching (Hogan, 1998).
Composting can be done in open system i.e. land treatment, and in closed system. The open land system can be inexpensive treatment method, but the temperature fluctuates from summer to winter. Therefore, rate of biodegradation of waste materials declines. Secondly, land treatment system may become oxygen limited, depending on amount of substrate, depth of waste, application, etc.
However, efficiency of open treatment system can be increased by passing air (Fig. 32.4). This approach is referred to as engineered soil piles and forced aeration treatment. The closed treatment system is preferred over the open land treatment system because controlled air is supplied to maintain the microbial activity.
As a result of microbial growth and volatilization of hazardous compounds, internal temperature gradually rises. Therefore, use of blowers for air circulation and exhaust for removal of toxic volatiles are set up on closed treatment system (Fig. 32.5). Ventilators supply oxygen and remove heat through evaporation of water.
Process of Composting:
As composting is a solid-phase biological treatment, target compounds must be either solid or a liquid associated with a solid matrix. The hazardous compounds should be biologically transformed. To achieve this goal, the waste material should be suitably prepared so that biological treatment potential should maximize. This is done by adjustment of several physical, chemical and biological factors (Fig. 32.6).
The hazardous wastes must be well solubilized so that they may be bioavailable. To hazardous compounds and soil organic matters serve the source of carbon and energy for microorganisms. Microbial enzymes secreted during growth phase degrade toxic compounds. However, proper maintenance of waste. O2, inorganic nutrients and pH increase the rate of decomposition.
If there is low substrate-density or site-specific conditions, analogue or non-analogue, non-hazardous carbon sources that can stimulate microbial growth and enzyme production can be added to compost. Organic amendment also stabilize microbial population in inhibitory environment.
Secondly, presence of sufficient amount of water enhances microbial growth. Addition of inorganic nutrients influences microbial growth and rate of decomposition of hazardous wastes. Under nitrogen limiting conditions Phanerochaete chrysosporium produces extracellular lignin peroxidase that degrades benzopyrene and 2, 4, 6-trinitrotoluene.
It has also been noted that a pH range of 5.0-7.8 promoted the highest rates of degradation of hazardous wastes. But lignin degradation has been found the most rapid at pH of 3.0-6.5. This shows that optimal pH levels can be species, site and waste specific.
Gradual colonization of organic materials is done by indigenous micro-flora, but hazardous chemicals may inhibit microbial growth. Therefore, bio-augmentation (i.e., use of commercial or genetically engineered microorganisms, i.e., GEMs) of wastes is also recommended.
To provide experimental proof of biodegradation during composting, a common hazardous contaminant pesticide, 14C-labelled Carbaryl was added in sewage sludge-wood chip mixture at 1.3-2.2. ppm concentration. After 18-20 days in laboratory composting apparatus, 1.6-4.9 per cent of Carbaryl was recovered as 14CO2 and remaining bound to soil organic matter.
Method # 3. Vermicomposting:
Vermicomposting is the phenomenon of compost formation by earthworms. Obviously, earthworms play an important role in the cycling of plant nutrients, turnover of organic matter and maintenance of soil structure. The earthworms can consume 10-20 per cent of their own biomass per day.
The most significant effect of earthworms in agro-ecosystems is the increase in nutrient cycling, particularly nitrogen. They ingest organic matter with a relatively wide C: N ratio and convert it to earthworm tissue with a lower C: N ratio. Thus, they affect the physico-chemical properties of soil.
In several countries including ours significant work has been done in this area. Scientists at Indian Institute of Sciences (Bangalore) have developed methods for frequent decomposition of coconut coir by using earthworms. Prof. B.R. Kaushal and coworkers at Kumaun University, Nainital have done significant work on earthworms, their food materials, food habit, organic matter turnover and have established relationships between food consumption, changes in work biomass, and casting activity of earthworms (Kaushal et al., 1994).
These fellows have also monitored the feeding and casting activity of Amuynthas alexandri on corn, wheat leaves and mixed grasses in laboratory cultures. Casts were produced in surface and sides of the containers. Food consumption varied from 36 to 69 mg/g live worm/day. Cast production ranged from 4 to 6 mg/g live worm/day (Kaushal et al., 1994).
Some of the known and potential waste decomposer earthworms (such as Drawidia nepalensis, etc.) may be introduced in such places where they are absent. Kausal and Bisht (1992) studied growth and cocoon production of D. nepalensis on urine-free cow and horse manure. D. nepalensis is slow growing vermicomposting species and also shows parthenogenesis. Its life cycle is given in Fig. 32.7.