The Different Types of Sewage Treatment

Breaking down raw sewage requires the addition of bacteria. Although the bacteria can be harmful to human health, they are beneficial to get the job done by reducing nutrient levels in the wastewater (or sewage) to eliminate toxicity before it can infiltrate the soil or contaminate water supplies. Sewage lagoons allow bacteria to flourish in a strictly controlled environment. There are three different ways used to encourage decomposition of waste material in a water treatment plant or sewage processing facility. They are anaerobic digestion, aerobic processing and artificial aeration. Often, more than one process is employed. Sometimes all three are used in series to treat raw sewage.

Anaerobic Digestion

Primarily used for processing animal waste rather than human sewage, anaerobic lagoons provide a more uniform mixture of water and waste for slow breakdown of the manure throughout varied layers of water and sludge. The sewage from residential and business users typically contains a more diverse collection of waste – food particles, soap residue and bits of non-biodegradable trash that can be strained out and subject to aerobic treatment. Anaerobic is defined as “without oxygen,” and this type of lagoon can be deeper with less surface area than aerobic lagoons. Covers are also commonly used to minimize air contact with the water to keep oxygen levels as low as possible. Anerobic ponds may require months to process manure and other animal waste to the extent that it can be discharged, but the process is totally passive and requires little maintenance other than periodic sludge removal.

Aerobic Processing

Always naturally or mechanically aerated in one of several different ways, aerobic processing relies on shallow water levels and a relatively high introduction of air into the water. Lagoons in naturally windy areas can process the water relatively swiftly and are usually unlikely to release unpleasant odors or dangerous gases. They do, however, require a large surface area and are not normally used for commercial facilities or livestock operations. It is virtually impossible to control seepage without an effective and reliable liner. Today, aerobic processing is almost always supplemented by artificial means.

Aeration Equipment

Pumps and automatic aerators are employed by municipal water treatment plants, often combined with odor-control procedures and covers that minimize the odors and gases produced as bacteria break down waste materials. The steady supply of oxygen to the water helps create an environment that can speed the breakdown of dangerous compounds and at least partially control the odor. Larger ponds consume a lot of energy to keep the water sufficiently aerated, but energy costs can be reduced with a two or three-stage systems of wastewater processing.

When employing any of the three types of holding or processing tanks or lagoons, the use of a reliable lining material is essential to prevent soil and groundwater contamination from the wastewater that is being treated. RPE geomembrane liners from BTL Liners are an effective safeguard to ensure that sewage lagoons and treatment plans operate safety and efficiently, no matter what type they are.

Wastewater Treatment Facilities

In most states, publicly owned wastewater treatment plants receive and process the sewage through a series of treatment steps that remove nutrients and solids, break down any organic materials and destroy existing pathogens in the water. Following such treatment, the “rejuvenated” water can be released into nearby streams, rivers or lakes, or it may be sprayed over large land areas.

The preliminary separation of large objects – including sticks, bottles, paper, rags, sand, grit or cinders – rapidly settle out of the liquid and are most often delivered to area landfills. Such objects rarely become part of the remaining sludge. In addition, because initial treatment utilizes gravity sedimentation to allow both organic and inorganic material to separate to the bottom of the container, most floating material like oil, grease, wood or vegetable matter can easily be skimmed from the surface and will not become part of the primary sludge.  

Secondary treatment is then carefully controlled and can be biologically accelerated to degrade (or digest) any dissolved organic material that is contained in the wastewater. It is typically converted into carbon dioxide which is released into the air and microbial cell mass that then settles to the bottom. This is known as secondary sludge.

Some treatment plants go further, employing a third step that reduces nitrogen and phosphorus content even more, and virtually eliminates suspended solids or biological oxygen demand in the water.  The remaining sludges are then normally combined; it is known at that point as “raw” sewage sludge, and it represents a potential health and environmental hazard. Additional treatment processes, are now available to stabilize the sludge, decrease its pathogen content and boost the solids content.

This wastewater treatment and sewage sludge information, prepared by the state of Pennsylvania, offers a thorough and highly technical discussion of modern techniques, processes and potential uses for the resulting treated sludge.


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