Water Quality Management 101
A basic overview of the key concepts related to treating municipal and industrial wastewater
Over the past few decades there has been an increased awareness of the importance of water quality. Many municipalities and industrial facilities have upgraded or installed new technologies to meet the demand for clean water. Advances in water quality science show that further improvements are needed to ensure a plentiful water supply and to protect the natural environment.
Water quality management is a complex field that requires participation by many diverse disciplines. People from all walks of life, including government workers, attorneys, engineers, scientists, business managers, educators, economists, politicians, environmental advocates, and the general public need to know more about water quality management.
Water pollution is a general term used to describe the degradation of water quality resulting from the loss of the productive or aesthetic uses of the receiving stream. Water pollution causes water quality impairment.
The primary causes of surfacewater pollution are from agriculture, municipal wastewater point sources, hydrologic modification (channelization, flow regulation, and dredging), urban runoff/storm sewers, and industrial point-source discharges.
Pollutants of concern include biochemical oxygen demand (BOD), total and volatile suspended solids, pH, toxic metals, oil and grease, nutrients, and specific organics.
Some pollutants can cause or contribute to more than one type of pollution, as indicated in Figure 1, including the following:
- Oxygen Depletion
- Temperature Effects
- Genetic Mutation
Wastewater Treatment Processes
Wastewater treatment technology is designed to reduce pollutants to acceptable levels as defined by the regulatory community.
Nearly 16,000 wastewater treatment plants are currently operating in the United States. They range in size from the 1 billion gallon-per-day (gpd) Chicago main wastewater treatment plant to smaller package plants with flow rates in the vicinity of 5,000 gpd or less. For all facilities, whatever the size, wastewater is treated in a train of sequential treatment processes, most often described as primary, secondary, and tertiary treatment.
Primary treatment removes many of the gross contaminants, such as grit, solids, and oil, so it will be more suitable for secondary treatment and to protect downstream equipment. Primary treatment steps include:
- Equalization of flow rate/volume and waste strength;
- Neutralization of acidic or alkaline wastewaters;
- Oil removal by gravity separation, dissolved air flotation, or other means;
- Suspended solids removal by sedimentation; and
- Heavy metals removal by chemical precipitation.
Secondary, or biological, treatment involves biological oxidation of organic and inorganic pollutants. The bulk of pollutants are usually removed during secondary treatment.
Biological systems are operated as aerobic, facultative, anaerobic, or anoxic reactors. In anaerobic reactors, the organics are converted to methane gas, which may be recovered and used as an energy source.
An aerobic system, like activated sludge treatment, removes most of the BODs treated in municipal and industrial wastewater treatment plants. The activated sludge process is relatively simple. After primary treatment, wastewater is directed to an aeration basin that is constructed of concrete or steel. Microorganisms are mixed with the wastewater and aerated in the basin where the organics are oxidized to carbon dioxide and water. The aeration basin's contents are subsequently moved to a clarifier where the biomass is settled and a portion re-circulated back to the aeration basin.
Tertiary treatment is applied to secondary effluents to improve effluent quality beyond the general secondary treatment standard. Tertiary treatment consists of physical/chemical treatment technologies designed to remove trace quantities of specific contaminants, including nutrients, total suspended solids, and refractory compounds.
Sludge Handling and Disposal
Most wastewater treatment processes generate sludges. These may consist of primary sludges (e.g., from municipal sewage plants and pulp and paper mills), or biological sludges generated in secondary treatment processes. Sludges usually undergo a series of treatment steps involving thickening, dewatering, and final disposal. Sludges may also undergo stabilization (for odor, pathogens, metals, etc.) prior to disposal.
Sludge disposal is a major problem for many wastewater treatment facilities. Sludges may be disposed by land application, land filling, and incineration.
Waste Minimization and Water Reuse
One of the main objectives of environmental engineering is to eliminate and minimize waste sources. A waste minimization program should be initiated before end of pipe wastewater treatment is implemented.
In arid areas, water reuse may be a necessity, while in other cases, water reuse should be considered as an economic option.
Economics of Wastewater Treatment
The economics of wastewater treatment is a primary consideration in the selection of wastewater treatment alternatives. A general procedure for the development of cost estimates includes the following:
- Collection of wastewater treatment data;
- Determination and selection of treatment processes;
- Determination of plant design size;
- Selection of unit process cost models; and
- Development of treatment cost estimates.
The ultimate goals of water quality management in the United States are the protection of human health and the environment, and the promotion of the nation's economic strength.
Understanding Water Quality Management: Technology and Applications is a comprehensive guide to the technologies of treating point-source wastewater.
- Comprehensive guide and reference to WQM by two leading authorities
- Explains aquatic ecosystems and industry pollutants that damage organisms
- Details the many causes of surface water quality deterioration
- Surveys a wide range of polluting elements, from metals to xenobiotics
- Offers specialized understanding of the sources, effects, and minimization of industrial pollution
- Shows the capital and cost effects of pollution control strategies
This article originally appeared in the 10/01/2005 issue of Environmental Protection.