What is Wastewater Treatment

 

The principal objective of wastewater treatment is generally to allow human and industrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment. Irrigation with wastewater is both disposal and utilization and indeed is an effective form of wastewater disposal (as in slow-rate land treatment). However, some degree of treatment must normally be provided to raw municipal wastewater before it can be used for agricultural or landscape irrigation or for aquaculture. The quality of treated effluent used in agriculture has a great influence on the operation and performance of the wastewater-soil-plant or aquaculture system. In the case of irrigation, the required quality of effluent will depend on the crop or crops to be irrigated, the soil conditions and the system of effluent distribution adopted. Through crop restriction and selection of irrigation systems which minimize health risk, the degree of pre-application wastewater treatment can be reduced. A similar approach is not feasible in aquaculture systems and more reliance will have to be placed on control through wastewater treatment.

Before you go on to read about the individual technologies discussed later in this document, it is helpful to understand some of the basics of wastewater treatment. You will see terms like BOD, total suspended solids, nitrification, and denitrification frequently when discussing wastewater treatment. It is important to understand what each of these terms mean and how each relates to the wastewater treatment process. Some very basic processes of wastewater treatment are also briefly discussed. If you understand the theory behind these basic treatment processes it is easy to see how and why the processes are applied in the various alternative technologies discussed later.

Basic Constituents of Wastewater

Biochemical oxygen demand

One of the most commonly measured constituents of wastewater is the biochemical oxygen demand, or BOD. Wastewater is composed of a variety of inorganic and organic substances. Organic substances refer to molecules that are based on carbon and include fecal matter as well as detergents, soaps, fats, greases and food particles (especially where garbage grinders are used). These large organic molecules are easily decomposed by bacteria in the septic system. However, oxygen is required for this process of breaking large molecules into smaller molecules and eventually into carbon dioxide and water. The amount of oxygen required for this process is known as the biochemical oxygen demand or BOD. The Five-day BOD, or BOD₅, is measured by the quantity of oxygen consumed by microorganisms during a five-day period, and is the most common measure of the amount of biodegradable organic material in, or strength of, sewage.

BOD has traditionally been used to measure of the strength of effluent released from conventional sewage treatment plants to surface waters or streams. This is because sewage high in BOD can deplete oxygen in receiving waters, causing fish kills and ecosystem changes. Based on criteria for surface water discharge, the secondary treatment standard for BOD has been set at 30 mg BOD/L (i.e. 30 mg of O₂ are consumed per liter of water over 5 days to break down the waste).

However, BOD content of sewage is also important for septic systems. Sewage treatment in the septic tank is an anaerobic (without oxygen) process; in fact, it is anaerobic because sewage entering the tank is so high in BOD that any oxygen present in the sewage is rapidly consumed. Some BOD is removed in the septic tank by anaerobic digestion and by solids which settle to the bottom of the septic tank, but much of the BOD present in sewage (especially detergents and oils) flows to the leaching field. Because BOD serves as a food source for microbes, BOD supports the growth of the microbial biomat which forms under the leaching field. This is both good and bad. On the one hand, a healthy biomat is desired because it is capable of removing many of the bacteria and viruses in the sewage so that they do not pass to the groundwater. The bacteria in a healthy biomat also digest most of the remaining BOD in the sewage. Too much BOD, however, can cause excessive growth of bacteria in the biomat. If the BOD is so high that all available oxygen is consumed (or if the leaching field is poorly aerated, as can be the case in an unvented leaching field located under pavement or deeply buried) the biomat can go anaerobic. This causes the desirable bacteria and protozoans in the biomat to die, resulting in diminished treatment of the sewage. Low oxygen in the biomat also encourages the growth of anaerobic bacteria (bacteria which do not require oxygen for growth). Many anaerobic bacteria produce a mucilaginous coating which can quickly clog the leaching field. Thus, excess BOD in sewage can cause a leaching field to function poorly and even to fail prematurely.

Many of the enhanced treatment technologies discussed later in this document were designed specifically to reduce BOD in treated sewage. BOD removal can be especially important where sewage effluent flows to a leaching field in tight soils. Tight soils are usually composed of silts and clays (particle size < 0.05 millimeter). These small soil particles are tightly packed and the pore space between them is small. Reducing BOD means that the sewage will support the growth of less bacteria and therefore the effluent will be better able to infiltrate tight soils. Many enhanced treatment technologies that remove BOD were designed specifically to enhance disposal of effluent in tight silt or clay soils.

BOD is fairly easy to remove from sewage by providing a supply of oxygen during the treatment process; the oxygen supports bacterial growth which breaks down the organic BOD. Most enhanced treatment units described incorporate some type of unit which actively oxygenates the sewage to reduce BOD. This unit is often located between the septic tank and the leach field. Or, it can be located within the septic tank in a specific area where oxygen is supplied. Reduction of BOD is a relatively easy and efficient process, and results in sewage of low BOD flowing to the leaching field. It is important to note, however, that low BOD in sewage may result in a less effective biomat forming under the leaching field.

It is also important to note that BOD serves as the food source for the denitrifying bacteria which are needed in systems where bacterially-mediated nitrogen removal takes place. In these situations BOD is desired, as the nitrification/denitrification process cannot operate efficiently without sufficient BOD to support the growth of the bacteria which accomplish the process.

Total suspended solids

Domestic wastewater usually contains large quantities of suspended solids that are organic and inorganic in nature. These solids are measured as Total Suspended Solids or TSS and are expressed as mg TSS/ liter of water. This suspended material is objectionable primarily because it can be carried with the wastewater to the leachfield. Because most suspended solids are small particles, they have the ability to clog the small pore spaces between soil grains in the leaching facility. There are several ways to reduce TSS in wastewater. The simplest is the use of a septic tank effluent filter, such as the Zabel filter (several other brands are available). This type of filter fits on the outlet tee of the septic tank. It is made of PVC with various size slots fitted inside one another. The filter prevents passage of floating matter out of the septic tank and, as effluent filters through the slots, fine particles are also caught. Many types of alternative systems are also able to reduce TSS, usually by the use of settling compartments and/or filters using sand or other media.

Total nitrogen

Figure 1. The Nitrogen Cycle

Nitrogen is present in many forms in the septic system. Most nitrogen excreted by humans is in the form of organic nitrogen (dead cell material, proteins, amino acids) and urea. After entering the septic tank, this organic nitrogen is broken down fairly rapidly and completely to ammonia, NH₃, by microorganisms in the septic tank. Ammonia is the primary form of nitrogen leaving the septic tank. In the presence of oxygen, bacteria will break ammonia down to nitrate, NO₃. In a conventional septic system with a well aerated leaching facility, it is likely that most ammonia is broken down to nitrate beneath the leaching field.

Nitrate can have serious health effects when it enters drinking water wells and is consumed. Nitrate and other forms of nitrogen can also have deleterious effects on the environment, especially in coastal areas where excess nitrogen stimulates the process known as eutrophication. For this reason, many alternative technologies have been designed to remove total nitrogen from wastewater. These technologies use bacteria to convert ammonia and nitrate to gaseous nitrogen, N₂. In this form nitrogen is inert and is released to the air.

Biological conversion of ammonia to nitrogen gas is a two step process. Ammonia must first be oxidized to nitrate; nitrate is then reduced to nitrogen gas. These reactions require different environments and are often carried out in separate areas in the wastewater treatment system.

The first step in the process, conversion of ammonia to nitrite and then to nitrate, is called nitrification (NH₃ NO₂+ NO₃). The process is summarized in the following equations:

Nitrification Process

It is important to note that this process requires and consumes oxygen. This contributes to the BOD or biochemical oxygen demand of the sewage. The process is mediated by the bacteria Nitrosomonas and Nitrobacter which require an aerobic (presence of oxygen) environment for growth and metabolism of nitrogen. Thus, the nitrification process must proceed under aerobic conditions.

The second step of the process, the conversion of nitrate to nitrogen gas, is referred to as denitrification. This process can be summarized as:

Denitrification

This process is also mediated by bacteria. For the reduction of nitrate to nitrogen gas to occur, the dissolved oxygen level must be at or near zero; the denitrification process must proceed under anaerobic conditions. The bacteria also require a carbon food source for energy and conversion of nitrogen. The bacteria metabolize the carbonaceous material or BOD in the wastewater as this food source, metabolizing it to carbon dioxide. This in turn reduces the BOD of the sewage, which is desirable. However, if the sewage is already low in BOD, the carbon food source will be insufficient for bacterial growth and denitrification will not proceed efficiently.

Figure 2. Denitrification

Clearly, any wastewater treatment unit that is going to remove nitrogen by the nitrification/denitrification process must be designed to provide both aerobic and anaerobic areas so that both nitrification and denitrification can proceed. As you look at the nitrogen removal technologies discussed later in this document, you will see how various designs have attempted to solve this problem in some unique and interesting ways.

Phosphorus

Phosphorus is a constituent of human wastewater, averaging around 10 mg/liter in most cases. The principal forms are organically bound phosphorus, polyphosphates, and orthophosphates. Organically bound phosphorus originates from body and food waste and, upon biological decomposition of these solids, is converted to orthophosphates. Polyphosphates are used in synthetic detergents, and used to contribute as much as one-half of the total phosphates in wastewater. Massachusetts has banned the sale of phosphate-containing clothes washing detergent, so phosphorus levels in household wastewater have been reduced significantly from previous levels. Most household phosphate inputs now come from human waste and automatic dishwasher detergent. Polyphosphates can be hydrolyzed to orthophosphates. Thus, the principal form of phosphorus in wastewater is assumed to be orthophosphates, although the other forms may exist. Orthophosphates consist of the negative ions PO₄^3-, HPO₄^2-, and H₂PO₄^–. These may form chemical combinations with cations (positively charged ions).

It is unknown how much phosphorus is removed in a conventional septic system. Some phosphorus may be taken up by the microorganisms in the septic system and converted to biomass (of course, when these microorganisms die the phosphorus is re-released, so there really is no net loss of phosphorus by this mechanism). Any phosphorus which is removed in the septic system probably is removed under the leaching facility by chemical precipitation.

At slightly acidic pH (as is found in the soils of Cape Cod and most of New England), orthophosphates combine with tri-valent iron or aluminum cations to form the insoluble precipitates FePO₄ and AlPO₄.

Insoluble Precipitates

Domestic wastewater usually contains only trace amounts of iron and aluminum. However, the sandy soil of Cape Cod frequently contains significant amounts of iron bound to the surface of sand particles. It is likely that this iron binds with phosphorus and causes some removal of total phosphorus below the leaching facility.

One caveat must need be added here. If the soil below the leaching facility becomes anaerobic, iron may become chemically reduced (changed to the Fe²⁺ form), which is soluble and able to travel in groundwater. In this case, the iron phosphate compounds may breakdown and phosphorus may also become soluble. Anaerobic conditions under the leaching facility can occur when the leaching facility is not well aerated, when there is a small vertical separation to groundwater, or when BOD in the sewage is so high that all oxygen present is depleted to oxidize BOD. In the conditions found on Cape Cod, the best method for maximizing phosphorus removal is probably to locate the leaching facility well above groundwater (>5 feet vertical separation) thereby providing a well-aerated area under the leaching field. To date, no alternative on-site technologies are capable of significant phosphorus removal. However, many are trying to achieve this goal and it is likely that within the next few years we may begin to see some technologies that are successful at phosphorus removal.

Basics of Sewage Treatment

The treatment of sewage is largely a biochemical operation, where chemical transformations of the sewage are carried out by living microorganisms. Different environments favor the growth of different populations of microorganisms and this in turn affects the efficiency, end products, and completeness of treatment of the sewage. Sewage treatment systems, whether they are standard septic systems or more advanced treatment technologies, attempt to create specific biochemical environments to control the sewage treatment process.

Three basic types of biochemical transformations occur as sewage is treated. The first is the removal of soluble organic matter. This is composed of dissolved carbon compounds such as detergents, greases, and body wastes, which make up much of the BOD content of the sewage. The second is the digestion and stabilization of insoluble organic matter. These are the sewage solids, such as body wastes and food particles, which make up the remainder of the BOD. The third is the transformation of soluble inorganic matter such as nitrogen and phosphorus.

The two major biochemical environments in which sewage treatment is carried out are termed aerobic and anaerobic environments. An aerobic environment is one in which dissolved oxygen is available in sufficient quantity that the growth and respiration of microorganisms is not limited by lack of oxygen. An anaerobic environment is one in which dissolved oxygen is either not present or its concentration is low enough to limit aerobic metabolism. The biochemical environment has a profound effect upon the ecology of the microbial population which treats the sewage. Aerobic conditions tend to support entire food chains from bacteria up to rotifers and protozoans. These microbes beak down organic matter using many metabolic pathways based on aerobic respiration with carbon dioxide as the main end product. Anaerobic conditions favor the growth of primarily bacterial populations and produce a different variety of end products, discussed below.

Anaerobic Digestion of Sewage

Solids in sewage contain large amounts of readily available organic material that would produce a rapid growth of microorganisms if treated aerobically. Anaerobic decomposition is able to degrade this organic material while producing much less (approximately one-tenth) biomass than an aerobic treatment process. The principal function of anaerobic digestion is to stabilize insoluble organic matter and to convert as much of these solids as possible to end products such as liquids and gases (including methane) while producing as little residual biomass as possible. It is for this reason that sewage treatment in a conventional septic tank is designed to be an anaerobic process. Organic matter treated anaerobically is not broken down to carbon dioxide; final end products are low molecular weight acids and alcohols. These may be further converted anaerobically to methane or, if sent to an environment (such as the leaching field) where aerobic bacteria are present, further broken down to carbon dioxide. Anaerobic digestion of organic matter is also a much slower process than aerobic digestion of organics and where rapid digestion of organic matter is needed an aerobic treatment process must be used.

As discussed above, an anaerobic environment is also necessary for denitrification, as the bacteria which carry out this process require anaerobic conditions to reduce nitrate to nitrogen gas. Many nitrogen-removal technologies are designed to provide an anaerobic treatment chamber as part of the treatment process.

Aerobic Treatment of Sewage

As the name implies, this process utilizes aerobic bacteria to break down sewage. The principal advantage of aerobic sewage treatment is its ability to rapidly and completely digest sewage, reducing BOD to low levels. Most of the alternative treatment technologies discussed in this document utilize some form of aerobic treatment of sewage. This process is used primarily to reduce BOD and, in systems that remove nitrogen, to nitrify the waste so that it can later be denitrified. Because the BOD in raw sewage is usually high, and available oxygen is rapidly consumed by the sewage, most aerobic treatment units are designed to supply supplemental oxygen to the sewage to keep the treatment process aerobic. Some units, such as the JET Aerobic system, use extended aeration to more completely digest the sewage solids. Most aerobic treatment units provide some type of artificial medium as a surface on which the sewage- digesting bacteria can grow. A variety of basic designs can be used for this purpose.

Attached culture systems are designed so that wastewater flows over microbial films attached to surfaces in the treatment unit. The surface area for growth of the biofilm is increased by placing some type of artificial media, such as foam cubes or various convoluted plastic shapes with high surface area, in the treatment chamber. This artificial media may sit in the treatment chamber with the effluent circulating through it, usually with supplemental air supplied so that treatment remains aerobic. This is the principal used by the JET Aerobic and FAST systems. Or, the media may be located outside the treatment chamber and wastewater is passed over the biofilm in intermittent doses. These designs are known as trickle filters and are one of the most common types of on-site treatment unit using attached cultures. Some technologies which employ trickle filters, and which are discussed in more detail later, include the Bioclere, Orenco trickle filter, and the Waterloo biofilter. Intermittent and recirculating sand filters, while located in separate chambers, can also be considered a form of trickle filter where sand is used as the media for bacterial growth. Because attached culture systems are generally aerobic, a complex community of microorganisms, including aerobic bacteria, fungi, protozoa, and rotifers, develops. These systems are capable of efficient removal of BOD. Being aerobic they will support the growth of nitrifying bacteria and can be used to nitrify wastewater, the first step in nitrogen removal.

Other aerobic systems utilize suspended culture of microorganisms to aerobically treat the sewage. This type of treatment assumes that a resident population of bacteria are present in the solids and sludge in the treatment unit; vigorous mixing of the sewage in the treatment compartment causes these bacteria to stay in suspension where they can aerobically digest the sewage. This principle is used by the Cromaglass and Amphidrome units as part of part of the batch reactor treatment process. It is also used in many large municipal sewage treatment plants.

The activated sludge process is similar to suspended culture in that it also utilizes the resident population of bacteria in the solids and sludge in the treatment unit, again, usually by mixing of the sewage so that the bacteria are kept in suspension. In the activated sludge process, however, there are usually periods where mixing ceases, and the solids are allowed to settle. It is then assumed that the sludge will become anaerobic and the anaerobic bacteria in the sludge will denitrify the waste. This is the principle used by batch reactors. As the name implies, batch reactors treat sewage in batches. A batch of sewage is allowed to settle so that solids are removed; the batch of sewage is then aerated and mixed and then allowed to settle for a period of anaerobic treatment (this process may be repeated several times on the same batch). When treatment is complete, the finished batch of sewage is pumped out and the next batch enters the unit to begin treatment. The Cromaglass and Amphidrome systems are examples of batch reactors.
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References

Grady, C.P. Leslie and Henry C. Lim, 1980. Biological Wastewater Treatment. Marcel Decker, Inc., N.Y

Peavey, Howard S., Donald R. Rowe, and George Tchobanoglous, 1985. Environmental Engineering, McGraw Hill Inc., N.Y.