The solids that result from wastewater treatment may contain concentrated levels of contaminants that were originally contained in the wastewater. A great deal of concern must be directed to the proper disposal of these solids to protect environmental considerations.
Failure to do this may result in a mere shifting of the original pollutants in the waste stream to the final disposal site where they may again become free to contaminate the environment.
A more reasonable approach to ultimate solids disposal is to view the sludge as a resource that can be recycled or reused.
All the sewage sludge produced at a treatment plant must be disposed of ultimately. Treatment processes such as have been described may reduce its volume or so change its character as to facilitate its disposal, but still leave a residue which in most cases must be removed from the plant site.
Like the liquid effluent from the treatment plant, there are two broad methods for the disposal of sludge - (1) disposal in water, and (2) disposal on land. This applies regardless of whether or not the sludge is treated to facilitate or permit the selected method of disposal.
1-Disposal in Water...
This is an economical but not common method because it is contingent on the availability of bodies of water adequate to permit it. At some seacoast cities, sludge either raw or digested is pumped to barges and carried to sea to be dumped in deep water far enough off shore to provide huge dilution factors and prevent any ill effects along shore.
In the past few years there has been an increased problem of pollutional loads, well above safe standards, affecting the south-shore beaches on Long Island, facilitating the closing of the beaches to the public. Some of these pollutional loads have been attributed to sludge deposits coming to shore form off-shore sludge barging operations.
Where barged to sea, the value of some treatment such as thickening or digestion, depends on the relative cost of the treatment and savings in cost by barging smaller volumes, or the value of gas produced by digestion.
2-Disposal on Land...
Under land disposal the following methods may be included :
- Application as fertilizer or soil conditioner.
This method is used principally for raw sludge, where, unless covered by earth, serious odor nuisances are created. The sludge is run into trenches two to three feet wide and about two feet deep. The raw sludge in the trenches should be covered by at least 12 inches of earth. Where large areas of land are available, burial of raw sludge is probably the most economical method of sludge disposal as it eliminates the costs of all sludge treatment processes. It is, however, rarely used and even then as a temporary makeshift because of the land area required. The sludge in the trenches may remain moist and malodorous for years so that an area once used cannot be reused for the same purpose or for any other purpose for a long period of time.
Use of sludge for fill is confined almost entirely to digested sludge which can be exposed to the atmosphere without creating serious or widespread odor nuisances. The sludge should be well digested without any appreciable amount of raw or undigested mixed with it.
Either wet or partially dewatered sludge, such as obtained from drying beds or vacuum filters can be used to fill low areas. Where wet sludge is used the area becomes a sludge lagoon, which has been discussed. When used as a method of disposal, the lagoon area is used only until filled, and then abandoned. When used as a method of treatment, the sludge after some drying, is removed for final disposal and the lagoon reused. Lagoons used for disposal are usually fairly deep. Sludge is added in successive layers until the lagoon is completely filled. Final disposal of digested sludge by lagoons is economical as it eliminates all dewatering treatments.
It is applicable, however, only where low waste areas are available on the plant site or within reasonable piping distance. They are frequently used to supplement inadequate drying bed facilities.
Dewatered digested sludge from drying beds and vacuum filters can be disposed of by filling low areas at the plant site or hauled to similar areas elsewhere without creating nuisances.
The ash from incinerators is usually disposed of by using it for fill. Where fill area is available close to the incinerator, the ash can be made into a slurry with water when removed from the ash hopper and pumped to the point of disposal. If the fill area is remote, the ash should be sufficiently wet to suppress the dust and transported by truck or railroad cars to the point of disposal.
* Soil Conditioning or Fertilizer...
Sewage sludge contains many elements essential to plant life, such as nitrogen, phosphorous, potassium, and in addition, at least traces of minor nutrients which are considered more or less indispensable for plant growth, such as boron, calcium, copper, iron, magnesium, manganese, sulfur, and zinc. In fact, sometimes these trace elements are found in concentrations, perhaps from industrial wastes, which may be detrimental. The sludge humus, besides furnishing plant food, benefits the soil by increasing the water holding capacity and improving the tilth, thus making possible the working of heavy soils into satisfactory seed beds. It also reduces soil erosion.
Soils vary in their requirements for fertilizer, but it appears that the elements essential for plant growth may be divided into two groups : those which come from the air and water freely and those which are found in the soil or have to be added at certain intervals. In the first group are hydrogen, oxygen and carbon. In the second group are nitrogen, phosphorous and potassium and several miscellaneous elements usually found in sufficient quantities in the average soil, such as calcium, magnesium, sulfur, iron, manganese, and others. The major fertilizing elements are nitrogen, phosphorous and potassium, and the amount of each required depends on the soil, climatic conditions and crop.
Nitrogen is required by all plants, particularly where leaf development is required. Thus, it is of great value in fertilizing grass, radishes, lettuce, spinach, and celery. It stimulates growth of leaf and stem.
Phosphorous is essential in many phases of plant growth. It hastens ripening, encourages root growth and increases resistance to disease.
Potassium is an important factor in vigorous growth. It develops the woody parts of stems and pulps of fruits. It increases resistance to disease, but delays ripening and is needed in the formation of chlorophyll.
Dried or dewatered sewage sludge makes an excellent soil conditioner and a good, though incomplete fertilizer, unless fortified with nitrogen, phosphorous and potassium. Head dried, raw activated sludge is the best sludge product, both chemically and hygienically, although some odor may result from its use. Heat dried, digested sludge contains much less nitrogen and is more valuable for its soil conditioning and building qualities than for its fertilizer content. For some crops it is deleterious. It is practically odorless when well digested.
Sludge cake from vacuum filters, because of its pasty nature, cannot be readily spread on land as a fertilizer or soil conditioner. It must be further air-dried. At some plants the sludge cake is stockpiled on the plant site over winter. Freezing, thawing and air drying result in a material which breaks up readily.
Digested sludge has been said to be somewhat comparable to farm manure in its content of fertilizer constituents, their relative availability and the physical nature of the material.
Before sludge digestion was so widely adopted, the application of raw sludge to fields was sometimes detrimental because the grease content was difficult for the soil to absorb and caused it to become impervious. In digested sludge, however, fat has been reduced and become so finely divided that it does not adversely affect the porosity of the soil.
The continued use of digested sludge tends to lower the pH value of soil and it is recommended that either lime or ground limestone be applied occasionally.
In some tests it has been found that activated sludge used as an organic carrier for added inorganic forms of nitrogen, has given better results for crops with a short growing season than activated sludge alone. The inorganic nitrogen is quickly available while that from the organic portion is available more slowly and lasts over a period of time.
In Binghamton, New York, the application of underwatered anaerobically digested sludge to crop land has produced a desirable corn crop for livestock feed.
There is a potential hazard of transmission of parasitic infections with air-dried digestion sludges as a result of handling the sludge or from sludge contaminated vegetables eaten raw. Spreading of digested sludge in the fall and allowing it to freeze in cold climates in the winter is believed helpful in killing these organisms. Heat-dried sludge is considered safe for use under all conditions because of the destructive action of heat upon bacteria.
In summary, it may be well to quote from Van Kleeck who has worked personally with sewage sludges and who has studied and written much on the subject :
Raw primary sludge, unless composted, is unsatisfactory as a soil conditioner because of its effect on the soil and on growing plants, and because of the health hazards involved.
Raw activated sludge, after heat drying, is established as a superior sludge product. Such sludge retains most of its organic solids and it contains more nitrogen than other sludges.
Digested sludges from all sewage treatment processes are materials of moderate but definite value as a source of slowly available nitrogen and some phosphorous. They are comparable with farm yard manure except for a deficiency of potash. Their principal value is the humus content resulting in increased moisture-holding capacity of the soil and a change in soil structure which results in a greater friability.
Composting can be defined as the aerobic thermophilic decomposition of organic wastes to a relatively stable humus. Decomposition results from the biological activity of microorganisms which exist in the waste. A good compost could contain up to 2 percent nitrogen, about 1 percent phosphoric acid, and many trace elements. Its most valuable features, however, are not its nutrient content, but its moisture retaining and humus forming properties.
Many types of microorganisms are involved in converting the complex organic compounds such as carbohydrates and proteins into simpler materials, but the bacteria, actinomycetes, and fungi, predominate. These organisms function in a composting environment that is optimized by copying the natural decomposition process of nature where, with an adequate air supply, the organic solids are biochemically degraded to stable humus and minerals.
Compost is generally considered as a material to be used in conjunction with fertilizer, rather than as a replacement for fertilizer unless it is fortified with additional chemical nutrients.
Compost benefits the soil by replenishing the humus, improving the soil structure, and providing useful nutrients and minerals. It is particularly useful on old, depleted soils and soils that are drought-sensitive. In horticulture applications, compost has been useful on heavy soils as well as sandy and peat soil. It has been commonly applied to parks and gardens because it increases the soil water absorbing capacity and improves the soil structure.
* Parameters : All composting processes attempt to create a suitable environment for thermophilic facultative aerobic microorganisms. If the environmental conditions for biological decomposition are appropriate, a wide variety of organic wastes can be composted. The most important criteria for successful composting are: (1) complete mixing of organic solids, (2) nearly uniform particle size, (3) adequate aeration, (4) proper moisture content, (5) proper temperature and pH, and (6) proper carbon-nitrogen ratio in the raw solids.
The smaller the particles, the more rapidly they will decompose; size is controlled by grinding. Air is necessary for aerobic organisms to function in a fast, odor-free manner. Aeration is enhanced by blending wastes to form a porous solids structure in the composting materials. Some composting systems use blowers while others aerate by frequent turning of compost placed in windows and bins. The solids to be composted must not, of course, contain high concentrations of materials toxic to the decomposing microorganisms.
A proper moisture content is the most important composting criteria. Microorganisms need moisture to function but too much moisture can cause the process to become anaerobic and develop the characteristic odor and slow decomposition rate associated with anaerobic processes.
Composting mixtures should have a pH near 7 (neutral) for optimum efficiency. The temperatures vary a great deal but those in the thermophilic range (greater than 110°F) produce a more rapid rate of decomposition than those in the lower mesophilic range. Higher temperatures also cause a more efficient destruction of pathogenic organisms and weed seeds.
An essential requirement of the composting process is control of the ratio of carbon to nitrogen in the raw materials. Microorganisms need both carbon and nitrogen, but they must be available in the proper amounts of decomposition will be prolonged.
The time required to complete composting varies, depending on the climate, materials composted, the degree of mechanization, whether the process is enclosed, and the desired moisture content of the final product. Composting detention times from a couple of weeks to several months have been reported.
*Composting Materials : Many types of wet solids have been successfully used in composting operations. These include sewage sludge, cannery solids, pharmaceutical sludge, and meat packing wastes. Sewage sludge has been frequently used as an additive when composting dry refuse and garbage. It enhances the composting operation because :
- It serves as a seeding material to encourage biological action.
- It helps to control the moisture content in the composting mixture.
- It enhances the value of the compost by contributing nitrogen and other nutrients.
- It can be used to control the important carbon/nitrogen ratio.
Normally, blending sewage sludge with other compost raw materials required prior dewatering of the sludge. If the dewatering step is omitted, the moisture content of the mixture is too high and odors develop. Reducing sludge moisture from 90 to 70 percent by vacuum filtration or centrifugation allows good aerobic composting with garbage at a blended moisture content of 53 percent.
In favorable climates, the composting of digested sludge with sawdust, straw, and wood shavings has been successful.