Wastewater Treatment Methods and Disposal - 1...
Wastewaters are waterborne solids and liquids discharged to the sewers and represent the wastewater of community life.
In composition wastewater includes dissolved and suspended organic solids, which are "putrescible" or biologically decomposable. Domestic wastewater also contains countless numbers of living organisms - bacteria and other microorganisms whose life activities cause the process of decomposition. When decay proceeds under anaerobic conditions, that is, in the absence of dissolved oxygen in the wastewater, offensive conditions result and odors and unsightly appearances are produced. When decay proceeds under aerobic conditions, that is, in the presence of dissolved oxygen, offensive conditions are avoided and the treatment process is greatly accelerated.
The overall philosophy of wastewater sanitation involving the removal, control and treatment of a wastewater in an area
that is isolated or remote from the center of activity is important. Over the years wastewater treatment management
practices have evolved into a technically complex body of knowledge based on past practice and applied engineering and environmental sciences. The intelligent application of these fundamentals goes a long way toward assuring us that the environment will be maintained in a safe and acceptable condition.
There are two general treatment objectives with respect to wastewater :
- Reducing or minimizing the public health hazards of a wastewater. These are general treatment measures aimed at preventing pathogens and other potentially harmful components from finding their way back to the consumer.
- Eliminating, reducing or minimizing the deteriorative impact of a wastewater on the receiving water quality and its environment.
A sharp distinction must be made between the term "wastewater disposal" and "wastewater treatment". All wastewater has to
be disposed of. Some wastewater is subjected to various types of treatment before disposal, but some wastewater receives no treatment before disposal.
Wastewater treatment is a process in which the solids in wastewater are partially removed and partially changed by
decomposition from complex highly putrescible organic solids to mineral or relatively stable organic solids. The extent of this change is dependent on the treatment processes involved. After all treatment processes have been completed, it is
still necessary to dispose of the liquid and the solids which have been removed.
There are three methods by which final disposal of wastewater can be accomplished. The general problem areas that are of concern in final disposal are pathogenic microorganisms (viruses, etc.), heavy metals and the presence of biologically resistant organic compounds, such as pesticides or insecticides which can find their way into water supplies. More
recently, there has been interest in the use of land for both surface and subsurface disposal after wastewater treatment.
Generally this is disposal by irrigation. This involves spreading the wastewater over the surface of the ground, generally
by irrigation ditches. There is some evaporation, but most of the wastewater soaks into the ground and supplies moisture
with small amounts of fertilizing ingredients for plant life. This method is largely restricted to small volumes of wastewater from a relatively small population where land area is available and where nuisance problems will not be created.
It has its best use in arid or semi-arid areas where the moisture added to the soil is of special value. If crops are cultivated on the disposal area, the growth of vegetation, often must be excluded from wastewater. Because untreated wastewater will also contain pathogenic organisms, the production of foods for human consumption which may be eaten without cooking is not desirable.
By this method wastewater is introduced into the ground below its surface through pits or tile fields. It is commonly used for disposal of settled wastewater from residences or institutions where there is only a limited volume of wastewater. Because it has little application for large scale use in municipalities.
Disposal by Dilution...
Disposal by dilution is the simple method of discharging wastewater into a surface water such as a river, lake, ocean, estuaries or wetlands. This results in the pollution of the receiving water. The degree of pollution depends on the
dilution, volume and composition of the wastewater as compared to the volume and quality of the water with which it is
mixed. When the volume and organic content of the wastewater is small, compared with the volume of the receiving water,
the dissolved oxygen present in the receiving water is adequate to provide for aerobic decomposition of the organic solids
in the wastewater so that nuisance conditions do not develop. However, in spite of the continued aerobic status of the
receiving water, microbial pollution remains a health menace and floating solids in the wastewater, if not previously removed, are visible evidence of the pollution.
Where the dissolved oxygen in the receiving water is inadequate to maintain aerobic decomposition, anaerobic decomposition takes place and putrefaction with objectionable conditions results. It is not so much the volume of wastewater that is the critical factor as the amount of readily decomposable organic matter in the wastewater. Thus a volume of of wastewater
that has been treated to remove or reduce this organic matter can be discharged to a natural surface water without creating objectionable conditions while the same volume of raw or untreated wastewater might produce a nuisance. The dissolved oxygen in the receiving water is the determining factor.
Less obvious problems associated with this type of disposal are the effects of toxic or potentially toxic compounds found
in domestic and industrial wastewater. These may involve immediate toxic effects such as heavy metals in fish and the "concentration" of certain biologically resistant compounds in the food chain. An example would be the accumulation of certain pesticides by microorganisms that are consumed by higher organisms to include fish, birds, and even man. Another subtle environmental effect now of some concern due to disposal of untreated wastewater by dilution is the enrichment of receiving waters by the introduction of plant nutrients such as nitrogen and phosphorous. The presence of excessive amounts
of these nutrients can stimulate plant and algae growth in the receiving waters. This is of special concern in inland, enclosed waters such as lakes and ponds, where the body of water can be harmed by gradual long-range changes that take place over the years.
Need for Wastewater Treatment...
The problem of wastewater disposal developed with man's use of water as a vehicle for carrying away the waste products of human life. Prior to that the volume of wastes, without the water vehicle, was small and disposal was largely restricted to the individual's or family's excreta. The earliest practice was simply to leave body waste and garbage on the surface of
the ground where it was gradually decayed by bacteria, mostly the saprophytic anaerobic type. This caused the production
of foul odors. Later, experience showed that if these wastes were promptly buried the odors could no longer be detected. Burial of human waste is a very ancient practice and even has biblical references. The next logical step was the development of the earth privy or outhouse, a method for the disposal of excremental wastes which is still widely used.
With urbanization and the development of community water supplies and the use of water to flush or transport wastes from habitations, it became necessary to find disposal methods not only for the wastes themselves, but for the water which
carried them. All of the three possible methods - irrigation, subsurface disposal and dilution - were employed.
As urban communities increased in population, with proportional increase in the volume of wastewater and in the amount of organic waste, all methods of disposal resulted in such unsatisfactory conditions that remedial measures became essential
and the development of methods of treatment of wastewaters prior to ultimate disposal was started.
The objectives originally sought in wastewater treatment include :
- Protection and maintenance of sources for use as domestic water supplies.
- Prevention of disease and spread of diseases.
- Prevention of nuisance conditions.
- Maintenance of clean waters for bathing and other recreational purposes.
- Protection and maintenance of the environment. For example, maintaining natural waters for the propagation and survival
of fish life.
- Conservation and protection of water for industrial and agricultural uses.
- Prevention of silting in navigable channels.
A wastewater treatment plant is designed to remove from the wastewater enough organic and inorganic solids so that it can
be disposed of without contravening or affecting the objectives sought.
Treatment devices merely localize and confine these processes to a restricted, controlled, suitable area or environment
and provide favorable conditions for the acceleration of the physical and biochemical reactions. The extent or degree of treatment needed varies greatly from place to place and is regulated by law. In general, the following are the determining factors :
- The character and amount of the solids carried by the wastewater, that is; BOD and suspended solids present.
- The objectives sought.
- The ability or capacity of the land (in disposal by irrigation and subsurface disposal) or the receiving water (in
disposal by dilution) to handle by self-purification or dispersal the water and solids in the wastewaters.
- Legal aspects and constraints.
The degree of wastewater treatment required to satisfy the first three conditions above is variable and is highly dependent on the local conditions and needs. Simple settling or even the mere removal of floating solids by screens may be adequate
for wastewaters under certain conditions, while a very high removal of suspended solids, decomposition of dissolved organic
solids and destruction of pathogenic organisms may be required before discharge to a river which is used downstream as a source of public water supply.
After the disposal of the wastewater effluent from a treatment plant, there still remains in the plant the solids and
water constituting the sludge which has been removed from the wastewater. This too must be disposed of safely and without nuisance.
The progress of self-purification of a stream can be measured by appropriate physical, chemical and biological laboratory tests. Similar tests are used to measure and control the progress of wastewater treatment plant processes.
The serious problem involving the disposal of wastewaters and other wastes by adequate and effective means that will eliminate nuisances and not violate the rights and welfare of individuals and communities has led to the development of laws and regulations governing such disposal.
It is presumed that in ancient times, customs slowly developed which regulated the disposal of the wastes of the individuals and of the group. As time went on, custom took on the force of law and led, over the years, to the formulation of legal
regulations - first as common law and then as statutory law.
Wastewater Treatment Methods and Disposal - 2...
Satisfactory disposal of wastewater, whether by surface, subsurface methods or dilution, is dependent on its treatment
prior to disposal. Adequate treatment is necessary to prevent contamination of receiving waters to a degree which might interfere with their best or intended use, whether it be for water supply, recreation, or any other required purpose.
Wastewater treatment consists of applying known technology to improve or upgrade the quality of a wastewater. Usually wastewater treatment will involve collecting the wastewater in a central, segregated location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment processes. Most often, since large volumes of wastewater are involved, treatment processes are carried out on continuously flowing wastewaters (continuous flow or "open" systems)
rather than as "batch" or a series of periodic treatment processes in which treatment is carried out on parcels or
"batches" of wastewaters. While most wastewater treatment processes are continuous flow, certain operations, such as
vacuum filtration, involving as it does, storage of sludge, the addition of chemicals, filtration and removal or disposal
of the treated sludge, are routinely handled as periodic batch operations.
Wastewater treatment, however, can also be organized or categorized by the nature of the treatment process operation being used; for example, physical, chemical or biological. Examples of these treatment steps are shown below. A complete treatment system may consist of the application of a number of physical, chemical and biological processes to the wastewater.
Some Physical, Chemical and Biological Wastewater Treatment Methods...
- Sedimentation (Clarification).
- Flotation and skimming.
- Ion Exchange.
- Activated sludge treatment methods.
- Trickling filtration.
- Oxidation ponds.
- Aerobic digestion.
- Anaerobic digestion.
- Septic tanks.
Physical methods include processes where no gross chemical or biological changes are carried out and strictly physical phenomena are used to improve or treat the wastewater. Examples would be coarse screening to remove larger entrained
objects and sedimentation (or clarification).
In the process of sedimentation, physical phenomena relating to the settling of solids by gravity are allowed to operate. Usually this consists of simply holding a wastewater for a short period of time in a tank under quiescent conditions, allowing the heavier solids to settle, and removing the "clarified" effluent. Sedimentation for solids separation is a
very common process operation and is routinely employed at the beginning and end of wastewater treatment operations. While
sedimentation is one of the most common physical treatment processes that is used to achieve treatment, another physical treatment process consists of aeration - that is, physically adding air, usually to provide oxygen to the wastewater. Still other physical phenomena used in treatment consists of filtration. Here wastewater is passed through a filter medium to separate solids. An example would be the use of sand filters to further remove entrained solids from a treated wastewater. Certain phenomena will occur during the sedimentation process and can be advantageously used to further improve water
quality. Permitting greases or oils, for example, to float to the surface and skimming or physically removing them from the wastewaters is often carried out as part of the overall treatment process.
In certain industrial wastewater treatment processes strong or undesirable wastes are sometimes produced over short periods of time. Since such "slugs" or periodic inputs of such wastes would damage a biological treatment process, these wastes are sometimes held, mixed with other wastewaters, and gradually released, thus eliminating "shocks" to the treatment plant. This is call equalization. Another type of "equalization" can be used to even out wide variations in flow rates. For example, the wet well of a pump station can receive widely varying amounts of wastewater and, in turn, pump the wastes onward at more uniform rates.
Chemical treatment consists of using some chemical reaction or reactions to improve the water quality. Probably the most commonly used chemical process is chlorination. Chlorine, a strong oxidizing chemical, is used to kill bacteria and to
slow down the rate of decomposition of the wastewater. Bacterial kill is achieved when vital biological processes are
affected by the chlorine. Another strong oxidizing agent that has also been used as an oxidizing disinfectant is ozone.
A chemical process commonly used in many industrial wastewater treatment operations is neutralization. Neutralization consists of the addition of acid or base to adjust pH levels back to neutrality. Since lime is a base it is sometimes used
in the neutralization of acid wastes.
Coagulation consists of the addition of a chemical that, through a chemical reaction, forms an insoluble end product that serves to remove substances from the wastewater. Polyvalent metals are commonly used as coagulating chemicals in wastewater treatment and typical coagulants would include lime (that can also be used in neutralization), certain iron containing compounds (such as ferric chloride or ferric sulfate) and alum (aluminum sulfate).
Certain processes may actually be physical and chemical in nature. The use of activated carbon to "adsorb" or remove organics, for example, involves both chemical and physical processes. Processes such as ion exchange, which involves exchanging certain ions for others, are not used to any great extent in wastewater treatment.
Biological Treatment Methods...
Use microorganisms, mostly bacteria, in the biochemical decomposition of wastewaters to stable end products. More
microorganisms, or sludges, are formed and a portion of the waste is converted to carbon dioxide, water and other end
products. Generally, biological treatment methods can be divided into aerobic and anaerobic methods, based on availability
of dissolved oxygen.
The purpose of wastewater treatment is generally to remove from the wastewater enough solids to permit the remainder to be discharged to a receiving water without interfering with its best or proper use. The solids which are removed are primarily organic but may also include inorganic solids. Treatment must also be provided for the solids and liquids which are removed as sludge. Finally, treatment to control odors, to retard biological activity, or destroy pathogenic organisms may also be needed.
While the devices used in wastewater treatment are numerous and will probably combine physical, chemical and biological methods, they may all be generally grouped under six methods.
Degrees of treatment are sometimes indicated by use of the terms primary, secondary and tertiary treatment. Tertiary treatment, properly, would be any treatment added onto or following secondary treatment.
At most plants preliminary treatment is used to protect pumping equipment and facilitate subsequent treatment processes. Preliminary devices are designed to remove or cut up the larger suspended and floating solids, to remove the heavy inorganic solids, and to remove excessive amounts of oils or greases.
To effect the objectives of preliminary treatment, the following devices are commonly used :
- Screens - rack, bar or fine.
- Comminuting devices - grinders, cutters, shredders.
- Grit chambers.
- Pre-aeration tanks.
In addition to the above, chlorination may be used in preliminary treatment. Since chlorination may be used at all stages in treatment, it is considered to be a method by itself. Preliminary treatment devices require careful design and operation.
In this treatment, most of the settleable solids are separated or removed from the wastewater by the physical process of sedimentation. When certain chemicals are used with primary sedimentation tanks, some of the colloidal solids are also removed. Biological activity of the wastewater in primary treatment is of negligible importance.
The purpose of primary treatment is to reduce the velocity of the wastewater sufficiently to permit solids to settle and floatable material to surface. Therefore, primary devices may consist of settling tanks, clarifiers or sedimentation tanks. Because of variations in design, operation, and application, settling tanks can be divided into four general groups :
- Septic tanks.
- Two story tanks - Imhoff and several proprietary or patented units.
- Plain sedimentation tank with mechanical sludge removal.
- Upward flow clarifiers with mechanical sludge removal.
When chemicals are used, other auxiliary units are employed. These are :
- Chemical feed units.
- Mixing devices.
The results obtained by primary treatment, together with anaerobic sludge digestion as described later, are such that they can be compared with the zone of degradation in stream self-purification. The use of chlorine with primary treatment is discussed under the section on Preliminary Treatment.
Secondary treatment depends primarily upon aerobic organisms which biochemically decompose the organic solids to inorganic
or stable organic solids. It is comparable to the zone of recovery in the self-purification of a stream.
The devices used in secondary treatment may be divided into four groups :
- Trickling filters with secondary settling tanks.
- Activated sludge and modifications with final settling tanks.
- Intermittent sand filters.
- Stabilization ponds.
The use of chlorine with secondary treatment is discussed under the section on Secondary Treatment.
This is a method of treatment which has been employed for many purposes in all stages in wastewater treatment, and even
prior to preliminary treatment. It involves the application of chlorine to the wastewater for the following purposes :
- Disinfection or destruction of pathogenic organisms.
- Prevention of wastewater decomposition : (a) odor control, and (b) protection of plant structures.
- Aid in plant operation : (a) sedimentation, (b) trickling filters, (c) activated sludge bulking.
- Reduction or delay of biochemical oxygen demand (BOD).
While chlorination has been commonly used over the years, especially for disinfection, other methods to achieve disinfection as well as to achieve similar treatment ends are also used. Among the most common is the use of ozone. In view of the
toxicity of chlorine and chlorinated compounds for fish as well as other living forms, ozonation may be more commonly used
in the future. This process will be more fully discussed in the section on disinfection.
The solids removed from wastewater in both primary and secondary treatment units, together with the water removed with them, constitute wastewater sludge. It is generally necessary to subject sludge to some treatment to prepare or condition it for ultimate disposal. Such treatment has two objectives - the removal of part or all of the water in the sludge to reduce its volume, and the decomposition of the putrescible organic solids to mineral solids or to relatively stable organic solids. This is accomplished by a combination of two or more of the following methods :
- Digestion with or without heat.
- Drying on sand bed - open or covered.
- Conditioning with chemicals.
- Vacuum filtration.
- Heat drying.
- Wet oxidation.
The term "package units" is used in the field to describe equipment which has been put on the market by a number of manufacturers that is intended to provide wastewater treatment by the use of prefabricated or modular units. Package
units can also refer to a complete installation, including both mechanisms and prefabricated containers. This term is also applied to installations where only the mechanisms are purchased and the containers constructed by the purchaser in
accordance with plans and specifications prepared by the manufacturer.
Though specific limitations have not been established, individual package units have, in general, been small installations serving a limited population.
Package units have been adapted to practically all the treatment devices, either singly or in various combinations that have been mentioned.
Tertiary and Advanced Wastewater Treatment...
The terms "primary" and "secondary" treatment have been used to generally describe a degree of treatment; for example, settling and biological wastewater treatment. Since the early 1970's "tertiary" treatment has come into use to describe additional treatment following secondary treatment. Quite often this merely indicates the use of intermittent sand filters for increased removal of suspended solids from the wastewater. In other cases, tertiary treatment has been used to describe processes which remove plant nutrients, primarily nitrogen and phosphorous, from wastewater.
Improvement and upgrading of wastewater treatment units as well as the need to minimize environmental effects has led to the increased use of tertiary treatment.
A term that is also sometimes used to indicate treatment of a wastewater by methods other than primary or biological
(secondary) treatment is advanced treatment. This degree of treatment is usually achieved by chemical (for example coagulation) methods as well as physical methods (flocculation, settling and activated carbon adsorption) to produce a high quality effluent water.