Module 5- Biological Treatment Techniques

Discussion Board | Supplemental Readingline

The objective of Module 5 is to introduce various biological treatment techniques for treating water and wastewater, including:

  • What is bacteria?
  • What are enzymes?
  • How are these used in wastewater treatment?
  • Attached growth systems
  • Suspended growth systems



Biological treatment consists of using microorganism and biochemical reaction or reactions to improve the water quality. Here, we will review the following biological treatment techniques:

  • attached growth processes
  • suspended growth processes



Bacteria may be aerobic, anaerobic or facultative. Aerobic bacteria require oxygen for life support whereas anaerobes can sustain life without oxygen. Facultative bacteria have the capability of living either in the presence or in the absent of oxygen. In the typical sewage treatment plant, oxygen is added to improve the functioning of aerobic bacteria and to assist them in maintaining superiority over the anaerobes. Agitation, settling, pH and other controllable are carefully considered and employed as a means of maximizing the potential of bacterial reduction of organic in the wastewater.

Single-celled organisms grow and when they have attained a certain size, divide, becoming two. Assuming an adequate food supply, they then grow and divide again like the original cell. Every time a cell splits, approximately every 20 to 30 minutes, a new generation occurs. This is known as the exponential or logarithmic growth phase. At the exponential growth rate, the largest number of cells are produced in the shortest period of time. In nature and in the laboratory, this growth cannot be maintained indefinitely, simply because the optimum environment of growth cannot be maintained. The amount of growth is the function of two variables: – environment and food. The pattern which actually results is known as the bacterial growth rate curve. Initially dehydrated products (dry) must first re-hydrate and acclimate in a linear growth phase before the exponential rate is reached.

Microorganisms and their enzyme systems are responsible for many different chemical reactions produced in the degradation of organic matter. As the bacteria metabolize, grow and divide they produce enzymes. These enzymes are high molecular weight proteins.

It is important to recognize the fact that colonies of bacteria are literally factories for the production of enzymes. The enzymes which are manufactured by the bacteria will be appropriate to the substrate in which the enzyme will be working and so you have automatic production of the right enzyme for the biological reduction of any waste material, provided you have the right bacteria to start with. Enzymes do not reproduce whereas as bacteria do.


Enzymes in biochemical reactions act as organic catalysts. The enzymes actually become a part of the action, but after having caused it, split off from it and are themselves unchanged. After the biochemical reactions are complete and products formed, the enzyme is released to catalyze another reaction. The rate of reaction may be increase by increasing the quantity of the substrate or temperature up to a certain point , but beyond this, the rate of reaction ceases to increase because the enzyme concentration limits it.

Bacteria & Enzymes in Waste Treatment Plants

All treatment plants should be designed to take advantage of the decomposition of organic materials by bacterial activity. This is something you can equate to lower costs, increased capacity, and an improved quality of effluent; even freedom from bad odors which may typically result when anaerobe bacteria become dominant and in their decomposition process, produce hydrogen sulfide gas and similar by-products.

Consider the fact that the total organic load of wastewater or sewage is composed of constantly changing constituent, it would be quite difficult to degrade all of these organics by the addition of one enzyme, or even several enzymes. Enzymes are specific catalysts and do not reproduce. What is needed is the addition of an enzyme manufacturing system right in the sewage that can be pre – determined as to its activity and performance and which has the initial or continuing capacity to reduce waste.

At the present time, the addition of specifically cultured bacteria seems to be the least expensive and most generally reliable way to accomplish desirable results. When you add the right bacteria in proper proportions to the environment, you have established entirely new parameters of potential for the treatment situation.

The following discussion outlines the biological process. This process is responsible for the digestion of organic waste, no matter where it occurs. With minor variations, this same process digest waste in:

Aquaculture, Agriculture, Composting, Livestock, Municipal, and Industrial Wastewater Treatment


Bacterial digestion is the process of bacteria, consuming organic matter. Enzymes act to break the organic matter into water – soluble nutrients, which the bacteria digest. Using complex chemical reactions, the organic waste is metabolized down to water and carbon dioxide (the final metabolic waste products), providing the bacteria with energy for growth and reproduction. It may be simply shown by the following equation:


Organic waste + water —–Enzyme—-> water soluble nutrients + oxygen —-Bacteria—> water + carbon dioxide


Organic waste + Water ——Enzyme——->Water Soluble Nutrients —–Bacteria—> Water + Carbon Dioxide

Simple Chemical Equation

Organic waste is consumed by the bacteria, used as nutrients by the bacteria, and is no longer present to produce odors, sludge, pollution, or unsightly mess.

Attached Growth Processes

Attached growth technologies work on the principle that organic matter is removed from wastewater by microorganisms. These microorganisms are primarily aerobic, meaning they must have oxygen to live. They grow on the filter media (materials such as gravel, sand, peat, or specially woven fabric or plastic), essentially recycling the dissolved organic material into a film that develops on the media.

In all cases, attached growth filters act as secondary treatment devices following a septic tank or other primary treatment. Raw wastewater must be treated first to remove the larger solids and floating debris, because these solids can plug the filter. There are two basic designs of attached growth or fixed film systems: those that hold the media in place, allowing the wastewater to flow over the bed (such as trickling filters), or those where the media is in motion relative to the wastewater ( e.g., rotating biological disks). In most cases, drains under the media collect the effluent and either send it back through the filter or send it on for further treatment. The main advantages of attached growth processes over the activated sludge process are lower energy requirements, simpler operation, no bulking problems, less maintenance, and better recovery from shock loads.

Attached growth processes in wastewater treatment are very effective for biochemical oxygen demand (BOD) removal, nitrification, and denitrification. Disadvantages are a larger land requirement, poor operation in cold weather, and potential odor problems.

Many small communities, housing developments, and individual homeowners are discovering that the use of filters or rotating biological contactors (RBCs), either alone or in some combination with other technologies, provide low-cost, low maintenance wastewater treatment.

Suspended Growth Treatment Systems

Suspended growth treatment systems freely suspend microorganisms in water. They use biological treatment processes in which microorganisms are maintained in suspension within the liquid. In suspended growth treatment systems, microorganisms convert the organic matter or other constituents in the wastewater into gases and cell tissue. The most common type of aerobic system is the suspended growth treatment system. Suspended growth technologies are conventional activated sludge treatment systems that use various process modes, ranging from conventional, extended aeration, contact stabilization, sequencing batch, and single sludge, which are available for polishing anaerobically treated effluents.

A water treatment system used with suspended growth treatment systems can apply to industrial plants, heating and cooling systems, cooling tower systems, steam systems, and domestic water supplies and swimming pools. A wastewater treatment system is an essential component of effluent distribution systems. These suspended growth treatment systems deliver wastewater to soil infiltrative surfaces either by gravity or by pressure distribution. Residuals are the by-products of a wastewater treatment process, including sludge and septage. Polymers are used for industrial water treatment as scale inhibitors in stressed cooling water. They do not contain phosphorous, inhibit the deposition of calcium carbonate and other low soluble salts, and provide good activity over a wide pH range.

Suspended growth treatment systems, such as biological treatment of water, involve the use of naturally occurring microorganisms in the surface water to improve water quality. Under optimum conditions, the organisms break down material in the water and improve the water quality. Natural suspended growth treatment systems, such as wastewater biological treatment, can be used for organic wastewaters such as municipal sewage and tend to be lower in cost for operation and maintenance. Although such processes tend to be land-intensive when compared to conventional biological processes, they are often more effective in removing pathogens and do so reliably and continuously when properly designed. Wastewater treatment equipment can be categorized as screening and conveying equipment, clarification and suspended solids removal equipment, aeration and biological treatment equipment, and tertiary treatment and disinfection equipment.



Continuous-Flow, Suspended-Growth Aerobic Systems (CFSGAS)


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