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Sewage Treatment

Sewage Treatment

Sewage treatment is designed to remove pollutants from water before returning it to the environment. Most of the solids, pathogens, and organic matter have been removed from water leaving a sewage treatment plant. Until recently, nitrogen removal has not been a matter of concern in the Long Island Sound region. Controlling nitrogen release means adding another level of treatment to the primary and secondary treatment for which most plants are designed.

In direct response to the LISS Interim Report, the states of New York and Connecticut in 1991 pledged to institute a no net increase policy, meaning that sewage treatment plants must not release more nitrogen than they have in the past. As land development and population increase, this policy alone will mean increasing the ability to remove nitrogen from sewage plant effluent.

Primary sewage treatment

Primary sewage treatment is a relatively uncomplicated physical process that mainly removes solids. The sewage first passes through screens that filter out large debris such as pieces of wood or cardboard. It then flows to a grit chamber where sand and other heavy particles are removed. Next, the sewage enters large sedimentation tanks where suspended solids slowly settle to the bottom and become sludge.

Primary sewage treatment removes 40%-60% of suspended solids and about 30% of organic matter. In plants that provide no further level of treatment, the water is chlorinated to kill any remaining pathogens and returned to the environment at this point. Primary sewage treatment alone is no longer considered sufficient, however.

Secondary sewage treatment

The federal Clean Water Act of 1972 mandates that all plants provide secondary sewage treatment. This same legislation requires that polluted industrial wastewater be pretreated before release to sewage treatment plants.

Secondary treatment begins where primary treatment leaves off. It is a more complicated and more costly process that accelerates the decay of organic matter by natural organisms. All but one of the 44 sewage treatment plants that empty directly into Long Island Sound are providing secondary treatment. Because of variations in design, the relation of capacity to flow, and the state of management, however, secondary treatment is not always fully achieved, especially in some older facilities.

In secondary treatment, the wastewater is exposed to a healthy population of oxygen-using decomposes bacteria and a plentiful supply of oxygen. For example, the water may be held in a tank where bubbles of air are blown through it and sludge is added to provide additional bacteria. Here, as for primary treatment, final chlorination assures that pathogens have been destroyed before water is released to the environment.

The result of effective primary plus secondary treatment is removal of about 90% of the organic matter, virtually all pathogens, and most solids. Between 10% and 20% of the nitrogen is also automatically removed because the decomposer bacteria require this much for their own growth.

Advanced sewage treatment

The potential pollutants remaining after secondary sewage treatment include heavy metals, nutrients, and nonbiodegradable organic chemicals such as PCBs or pesticides. "Advanced sewage treatment" is a general term covering treatment designed to remove any of these substances.

A variety of types of advanced treatment are available for nitrogen removal. Some are better proven than others and some are much more expensive than others. The possibilities for additional nitrogen removal by sewage treatment plants depend on their size and design. Plants that have additional capacity and are appropriately designed can alter their treatment process so that nitrogen is removed by the method known as biological nutrient removal, which is described in the next section. The LISS interim report recommends that all plants with the potential to make the necessary minor changes do so. This option is available for fewer than half of the plants that discharge directly into the Sound or into waterways that flow to the Sound. In Connecticut, the state government is requiring plants to undergo these changes and is paying for them.

Specific recommendations for the more costly reconstruction needed by undersized plants to improve nitrogen removal will be included in the forthcoming LISS Comprehensive Conservation and Management Plan. Such reconstruction will cost billions of dollars. The comprehensive plan will also include recommendations for the reduction of nitrogen from nonpoint sources of pollution. A number of actions that individual homeowners can take immediately are covered in following sections of the Soundbook.

Biological nutrient removal

Biological nutrient removal (BNR) resembles secondary sewage treatment in using natural microorganisms to do the job. The difference is that the single zone for aeration of wastewater is replaced with two zones. One has ample oxygen (aerobic) and one has almost no oxygen (anoxic).

Nitrogen in sewage is mostly present as ammonia (the same substance that is in household ammonia) or in organic matter from plants and animals. With oxygen available, bacteria convert this nitrogen to nitrate by combining it with oxygen. The process is called nitrification, and it occurs both in nature and in secondary sewage treatment.

When no oxygen is available, other kinds of bacteria in sewage meet their oxygen needs by taking it away from nitrate. In this process, called denitrification, nitrate is converted to harmless nitrogen gas that bubbles out of solution and into the atmosphere. Denitrification also occurs under the low-oxygen conditions in flooded wetlands, providing another good reason for wetlands protection.

By alternately holding wastewater in aerobic and anaerobic zones within a sewage treatment plant, about 90% removal of the nitrogen can be achieved. This stage of treatment can only be added to existing plants if they are operating under their design capacity and therefore have extra space available.

[For those curious about chemical formulas:

  • Ammonia in water solution is present as NH3
    and NH4+.
  • Nitrate is N03-,
  • Nitrite is N02-.
  • Nitrogen gas is N2.
  • (N represents nitrogen, H represents hydrogen, 0 represents oxygen)]
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