The liquid wastes can be broadly divided into two groups:
i. Biological wastes
ii. Non-biological wastes
The biological liquid wastes, for example, municipal sewage, wastes from tanneries, paper mills, sugar mills, distilleries, food manufacturing units, etc. contain organic matters that are degradable in nature.
The non-biological liquid wastes comprise of inorganic substances that are non-degradable. Such wastes include solids and liquids in suspended or dissolved form, which originate from chemical industries, fertilizer plants, cement factories, etc.
Liquid wastes must be treated before being disposed of otherwise the pollutants will contaminate the environment and pose a health hazard. Since the nature and composition of liquid wastes varies different treatments are required before they can be safely disposed of Sewage or any other organic waste is treated in three stages primary, secondary and tertiary treatments.
Disposal of Gaseous Wastes:
The gaseous wastes are the main source of air pollution. As air pollution is most dangerous to human health, the gaseous wastes must be properly treated before they are disposed of into the atmosphere. The following techniques are undertaken for the management and disposal of gaseous wastes.
Filters are employed to remove the particulate matter in the gaseous wastes. The filters remove the particles by trapping them into porous bags which allow only air to pass through. Such filters are also called bag filters. The bags are prepared from cotton, polyester, spun glass fibers, etc. The bags are 10-15 m long and 2-3 m wide. When the gaseous wastes are blown into the bottom of the bag, gases escape through the pores and the particulate matter is trapped. After regular intervals the bags are cleaned to remove the accumulated particles. Such filters are commonly used in textile, glass and asbestos industries.
Scrubbers are devices that are installed to separate particulate matter as well as gaseous pollutants. It consists of a device in which the gaseous wastes are passed through a space containing wet packing. Therefore, these are also called wet scrubbers (Fig. 17.5). Water is most commonly used for wet packing, while some other liquids may also be used to remove the pollutants.
The solid particles as well as the gaseous pollutants are adsorbed on the wet packing. Though, scrubbers are highly efficient as they remove 80 to 90% of the hazardous wastes, they are quite costly as they consume water and electricity. A cost effective method is to install spray towers where the gaseous pollutants are sprayed with water to remove the gaseous and particulate contaminants.
Electrostatic precipitators (ESP) are electrical devices for controlling particulate matter from smoke and other gaseous pollutants. The gaseous wastes are passed through a collection plates at high voltage (30,000 to 1, 00,000 volts) whereby negative electrical charge is imparted to the suspended particles. These negatively charged particles migrate towards the collection plates which are positively charged.
During collection, the charge is neutralized and the particles are removed by washing or by gravity action. Electronic precipitators are extremely efficient (99%) as they can remove micron size particles and are also simple to maintain. However, these precipitators consume large amount of electricity.
Air pollution from the automobiles is quite common in the urban areas. Automobiles emit carbon monoxide, hydrocarbons, etc. that are a constant health hazard. The control of these gases is of considerable importance, not only for human health, but also for controlling the greenhouse effect and ozone layer depletion. Catalytic converter is the most common device installed in the automobiles that convert carbon monoxide to carbon dioxide and hydrocarbons to carbon dioxide and water.