ENVIRONMENTAL MANAGEMENT OF WASTE-TO-ENERGY PROJECTS
Any waste handling, treatment and disposal facility, either for energy/ resource recovery (including compost) or only for waste destruction, can be a source of environmental pollution (air/ ground/ water and land / visual/ noise/ odour pollution/ explosion), unless proper measures are taken in its design and operation.
The major environmental concerns in case of the Waste-to-Energy facilities based on the established technologies of Incineration and Anaerobic Digestion, and the control measures necessary, are discussed in the next Section. The concerns/ measures for Incineration are, however, generally also applicable to the other waste processing/ treatment methods involving thermo-chemical treatment and those for Anaerobic Digestion, to the other waste processing/ treatment methods involving bio-chemical treatment (including composting).
15.5.1 Environmental Pollution Control (EPC) Measures for Incineration Plants
Incinerators burning MSW can produce a number of pollutants in the flue gas in varying concentration like carbon monoxide, sulfur dioxide, and particulate matter containing heavy metal compounds and dioxins. Many of these pollutants are formed as a result of incomplete/ partial combustion. That is, refuse that is not burned at high enough temperatures, for long enough or when too much or too little air has been added to the fire. The generation of these pollutants and their release into the atmosphere can be effectively reduced or prevented by incorporating a number of air pollution control devices and by proper operation of the WTE facility.
Concentrations of heavy metals in particulates, particularly lead, zinc, mercury and cadmium, may be significant and care must be exercised in their removal and disposal. The most important of flue gas pollutants are sulphur dioxide (SO2) and hydrogen chloride (HCl), the agents of acid rain. These may be eliminated by wet scrubbers. Hydrogen fluoride and oxides of nitrogen are also produced but are not normally a problem because of low concentrations. The emission of combustible, carbon-containing pollutants- dioxins and furans, is also of concern. The same can be controlled by optimizing the combustion process.
Other concerns related to incineration include the disposal of the liquid wastes from floor drainage, quench water, and scrubber effluents, and the problem of ash disposal in landfills because of heavy metal residues.
Following is a summary of the gaseous emission control devices now being used to remove pollutants from incinerator stack:
Dry Scrubbers
These “wash” particulate matter and gases from the air by passing them through a liquid. The scrubber removes acid gases by injecting a lime slurry (a watery mixture) into a reaction tower through which the gases flow. A dry powder containing salts is produced and collected alongwith the fly ash in an electrostatic precipitator or in filters and discharged alongwith the fly ash into the ash residue.
The lime also causes small particles to stick together, forming larger particles that are easier to remove. Ash is stabilized by the addition of lime which enhances its natural alkalinity.
Electrostatic Precipitators (ESP)
These units use high voltage to negatively charge incoming dust particles, then the charged particles are collected on positively charged plates, ESPs - documented as removing 99.95% of Total Suspended Particulates (TSPs), including heavy metals - are very commonly used as WTE air pollution control devices. Nearly 43% of all existing facilities use this method to control air pollution.
Fabric Filters (Bag houses)
These consist of hundreds of long fabric bags made of heat-resistant material suspended in an enclosed housing which filters particles from the gas stream. Fabric filters are able to trap fine, inhalable particles ( <10 microns) and can capture 99% of the particulates in the gas flow coming out of the scrubber, including condensed toxic organic and heavy metal compounds.
Stack Height
Stack height is an extra precaution taken to ensure that any remaining pollutants will not reach the ground in a concentrated area. When the gasses enter the stack they are quite clean due to the controls discussed above. Stacks being built today are 200-300 feet (60-90m)or more in height, twice as high as the stacks used on older municipal incinerators. Stack heights should be determined by calculating quantity of fuel used and considering local weather conditions also. Standard equations could be used for determining stack heights.
EPA, USA has developed strict air emission standards for incinerators. CPCB has also stipulated certain standards for medical waste incinerators and these standards could be enforced till specific standards are evolved for MSW incinerators.
The schematic of a typical flue-gas cleaning system of a modern MSW combustion facility is give n in Fig. 15.14.
