NOTATIONS
2.3 COMPOSTING
2.3.1 Types of composting
According to Haug (1993) the composting process can be achieved by the different methods as represented in Fig. 2.4
• Open process
o Windrow process
Piling of the biodegradable wastes in long rows refers as windrow composting.
This method is most suited for large volumes of wastes. Using specific turners to improve aeration and porosity or to remove excess moisture content usually turns the windrows. The temperature of the windrows must be measured and logged constantly to determine the optimum time to turn them for quicker compost production. Typically, the shapes of the windrows are trapezoidal and size varies from 90 to 360 cm in height, and width from 300 to 600 cm. The time required to achieve good quality compost depends on type of wastes but usually it takes 90-270 days to complete the process. It is commonly used farm scale composting methods.
Fig. 2.4. Composting methods
o Passive aerated pile
Air is contributed to the composting feedstocks via perforated pipes inserted under each pile, thus eliminating the need for turning. The pipe ends are open.
Composting
Open Process
Windrow Static Pile
Aerated Pile
In-Vessel Process
Horizontal Flow Vertical Flow
The principal behind passive aerated pile that air flows into the perforated pipes and through the pile because of the chimney effect built as the hot gases rise upward out of the pile. The pile should be 90-120 cm high. The FAO (2003) reported the holes drilled in the pipes should be about 1.27 cm diameter. When the composting period is completed, the pipes are removed, and the base material is mixed with the compost. This method has been studied and used in Canada for composting seafood wastes with peat moss, manure slurries with peat moss, and solid manure with straw or wood shavings. The time required to achieve good quality compost depends on type of wastes but usually it takes 45-90 days to complete the process.
Fig. 2.5 Windrow Process o Active aerated pile
In this type of composting method, the blended feedstock’s is placed on the perforated piping’s, and for air circulation pipes are connected to the blower. In large scale composting systems, force aeration is performed by the computerized monitoring system. Depending on the substrates porosity, environmental conditions the height of the aerated pile systems should be maintain between 150- 245 cm whereas the width varies about 300-490 cm generally triangular in shape.
• In-vessel process
In-vessel process refers to a group of methods that confine the composting materials within a container or vessel (NRAES, 1992). In-vessel methods rely on a different techniques to expedite the composting process. Wide variety of in-vessel methods either individual or with different combinations of vessels, aeration devices, and turning mechanisms utilized by many researchers in past. Most of the techniques were used to manage municipal solid waste, including final treatment of sewage biosolids, to a safe stable state for reclamation as a soil amendment. In general, in-vessel process are usually decentralized systems where small scale composting can be performed in batch or continuous modes.
a) Passive aerated pile b) Active aerated pile Fig. 2.6. Pile composting (Rynk, 1992)
Horizontal and vertical reactors are commonly referred in-vessel systems. Vertical composting reactors (Fig. 2.7) are generally over 4 meters (yards) high, and can be housed in silos or other large structures. Waste is typically fed into the reactor from the top through a distribution mechanism, and it flows by gravity at the bottom. The height of these reactors makes process control difficult due to the high rates of airflow required per unit of distribution surface area. Neither temperature nor oxygen can be maintained at optimal levels throughout the reactors, leading to zones of non-optimal activity.
Horizontal reactors avoid the high temperature, oxygen, and moisture gradients of vertical reactors by maintaining a short airflow pathway (Fig 2.7). Agitated systems usually use the turning process to move material through the system in a continuous mode, while static systems require a loading and unloading mechanism.
Materials handling equipment may also shred to a certain degree, exposing new surfaces for decomposition, but excessive shredding may also reduce porosity. Aeration systems are usually set in the floor of the reactor, and may use temperature and/or oxygen as control variables. Systems with agitation and bed depths less than two to three meters (yards) appear effective in dealing with the heterogeneity of MSW.
Fig. 2.7 Vertical Reactors
Fig.2.8 Rotary drum composter (Rynk, 1992)
An efficient and promising technique of decentralized composting is the rotary drum composter. Rotary drum provides agitation, aeration and mixing of the compost, to produce a consistent and uniform end product. The composting time is drastically
reduced to 2–3 week. Rotary drum of medium sizes can also be placed at waste generation sites. Different types of waste (cow dung, swine manure, municipal bio- solids, brewery sludge, chicken litter, animal mortalities and food residuals) can be decomposed effectively in rotary drum.
Rotary drum (horizontal flow reactor) employs a bin of varying geometry and method of agitation. There are critical types of rotary drum, which differ in geometry, and method of agitation (Haug, 1993). The most extensively used type of rotary drum is Dano Drum. Dano Ltd. in Denmark developed it in 1933 for composting refuse. It dealt with high rate composting of municipal solid waste. However, information on operational aspects and compost dynamics for the mixed organic wastes in a rotary drum composter is rather limited.