• Application as fertilizer or soil conditioner
Burial 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.
The option of using 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. 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. It should be clear to you that the above options for sludge sill are temporary solutions, and they still have environmental trade-offs. In the end, they too represent environmentally unfriendly solutions and are end-of-pipe disposal technologies that add costs to treatment.
Well what about pollution prevention type technologies? The two we will explore in some detail are Soil Conditioning or Fertilizer and Composting.
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.
There is a potential hazard of transmission of parasitic infections with air-dried digestion sludge 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.
Let's talk about composting. A good compost could contain up to 2 percent
O 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. O Raw activated sludge, after heat drying, is established as a superior sludge product. Such sludge retains most of Us organic solids and it contains more nitrogen than other sludge.
O Digested sludge 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.
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.
Composting is the process of aerobic thermophilic decomposition of organic wastes to a relatively stable humus. Decomposition results from the biological activity of microorganisms which exist in the waste.
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.
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: (1) it serves as a seeding material to encourage biological action, (2) it helps to control the moisture content in the composting mixture, (3) it enhances the value of the compost by contributing nitrogen and other nutrients, and (4)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.
The balance of our discussions focus on the pollution prevention technologies for sludge management and use. When you surf the Internet or look at some of the technical and trade journal references, you will more commonly see these subjects referred to as sludge and biosolids resuse.
As we have seen, sewage sludge has many characteristics that are good for soils and plants, if applied properly. Research has shown that the organic matter in sludge can improve the physical properties of soil. Reused sludge is also considered biosolids, which is a slightly more attractive name ~ don'tyou think? Used as a soil additive, sludge improves the bulking density, aggregation, porosity and water retention of the soil. When added properly, sludge enhances soil quality and makes it better for vegetation. Vegetation also benefit from the nitrogen, phosphorus and potassium in sludge. When applied to soils at recommended volumes and rates, sludge can supply most of the nitrogen and phosphorus needed for good plant growth, as well as magnesium and many other essential trace elements like zinc, copper and nickel.
There are alternative systems to the marketability of biosolids from wastewater treatment plants. In fact, there are more than a dozen systems encompassing Class A pathogen-reduction technologies, but among these the most promising and widely used are alkaline stabilization, thermal drying, and composting.
We only briefly mentioned alkaline stabilization, but in reality this is a variation of sludge pasteurization. The basic process uses elevated pH and temperature to produce a stabilized, disinfected product. The two alkaline stabilization systems most common in the U.S. are a lime pasteurization system and a cement kiln dust pasteurization system. The lime pasteurization product has a wet-cake consistency, while the kiln dust pasteurization has a moist solid like consistency. Both products can be transported to agricultural areas for ultimate use. Literature studies show that the kiln dust product can capture a marketable value of $6.60/Mg ($6.00/ton) to offset hauling costs, while the lime product does not appear to be able to capture financial credits for product revenues at this point in time. The reasons for this are not entirely clear.
In contrast, composting processes utilize a mixture of solids and yard waste under controlled environmental conditions to produce a disinfected, humus-like product. Three common composting systems are a horizontal agitated reactor, a horizontal nonagitated reactor, and an aerated static pile system (nonproprietary). Compost can be marketed as a soil conditioner in competition with such products as peat, soil, and mulch. Although a large potential market exists, significant effort is required to penetrate this market. Yard waste revenue of $6.50/m3 (S5/yd3) and product revenue of $2.00/m3 ($1.50/yd3) appear to be reasonable market values based on various studies reported on the Web.
The lime stabilization system has advantages of low capital costs, process reliability, flexibility, and operability. The main disadvantage attributed to this system are questionable product marketability because of the uncertain availability of suitable agricultural land in some parts of the country where the product could be locally marketed. The steam drying alternative has the advantages of small facility land requirements, good public acceptance, and favorable product marketability. The disadvantages of this system included relatively high capital costs, reduced expansion flexibility, and complex operational requirements. These advantages and disadvantages apply to all of the thermal drying alternatives. Land application is the largest beneficial use for sewage sludge. Since municipal sludges are a by by-product of the foods we eat, they contain important nutrients such as nitrogen, phosphorus and potassium. Proper land application provides a way to recycle these nutrients and return them to the soil safely. Sludge can also be processed into heat dried pellets that are marketed as fertilizers and soil conditioners. The peptization process also reduces disease causing organisms. Golf courses, parks, cemeteries, nurseries and municipal landscaping projects provide markets for such pelletized sludge products.
Composting is another way to recycle nutrients and organic matter in sludge. The benefits from using sludge composts include increased water and nutrient holding capacity and increase aeration and drainage of soils. Composted sludge's also provide the soil with low levels of plant nutrients. Sludge compost is currently being produced and marketed by municipalities around the United States. More and more cities are turning to composting as a method to beneficially manage sludge's.
There are concerns that land application of sludge will result in an increase of pathogenic bacteria, viruses, parasites, chemicals and metals in drinking water reservoirs, aquifers, and the food chain. This raises additional concerns of cumulative effects of metals in cropped soils. Research shows that if metals such as zinc, copper, lead, nickel, mercury, and cadmium are allowed to build up in soils due to many applications of sludges over the years, they could be released at levels harmful to crops, animals, and humans. While some of these metals are necessary micronutrients, at higher levels they may be harmful to plants, particularly those grown on acid soils (soils with a low pH). Cadmium, a suspected carcinogen, and mercury cause even greater concern because of their toxic effects on animals and humans. Likewise, synthetic organic compounds such as dioxins and PCBs, if present, cause concern about ecological and human health impacts. The degree of risk depends directly on the initial sludge quality, the way the sludges are processed and how the amended soil is managed during and after land application. Current state and federal legislation requires sludge treatment processes to reduce pathogens prior to land application. Furthermore, state and federal standards mandate specific limits for metals contained in sludge. Since metal concentrations depend mainly on the type and amount of industrial waste that flows into the wastewater treatment system, strongly enforced pretreatment and source control programs could effectively reduce the metals content of sludge. Providing proper employee training and applying the best management practices will yield the best sludge use program. The fate of sludge components is also influenced by factors such as climate (rainfall and temperature), soil management (irrigation, drainage, liming, fertilization, and addition of amendments), and composition of the sludge. In the past, the success of land application has been hurt by the mismanagement of important factors such as soil pH. For example, the uptake of many metals, such as cadmium, is related to soil pH. If pH drops below a certain level, heavy metals will be released, increasing the chances of leaching and plant uptake. In addition, nutrient contamination of surface waters through nonpoint source pollution needs to be carefully monitored. While not a concern for human health and the environment, odors associated with poorly managed sludge
Philadelphia and Washington D.C. both market sludge compost for use as a mine spoils cover, landscaping material, soil amendment for public lands, and potting material. Sludge composts can also be used along roadsides to establish vegetation and reduce erosion, uses which require only a single or infrequent permit application.
application can be a serious concern to those living near application sites. Prompt incorporation of sludges and sludge products into the soil and avoidance of stockpiling can help to prevent odor problems. It is essential for sludge management programs to have knowledgeable staff available to teach people how to apply and monitor the sludges and the treated area correctly.
In general, researchers agree that the effects of organic compounds, certain pesticides and metals are not dangerous when managed properly at regulated levels. However, they caution that additional study of organic compounds and long-term fate of materials is needed before unlimited application of sludge can occur safely on all lands.
Sludge landfill can be defined as the planned disposal of wastewater solids including sludge , grit, and ash at a designated site where it is buried and monitored. The sludge is delivered to the landfill by trucks that pick up the sludge from the wastewater treatment plants. There are several different types of landfilling, these are all listed below under disposal methods, but the most frequent method used is dewatering then burial. This method is done by the plant dewatering the sludge then trucks pick up the sludge which is approximately 80% moisture and 20% solids. The trucks then dump the sludge into the landfill, where tractors bury the sludge using one of two special burial techniques. These techniques utilize space most efficiently and develop a grade for drainage of precipitation.
Many municipalities and state regulatory agencies do not want sludge to be landfilled. Most states require special permission to do so. Landfills must be monitored regularly with monitoring wells and a few other environmental safety measures. The municipality are the state determine where and how the sludge will be disposed of. Once they are designated to be a part of land use, the sludge is either landfilled or if it is usable or the right grade, which is usually grade A, the sludge is used for composting. The essential difference between land application and landfill is that land application leads to treatment or assimilation, while landfill leads to containment and only for an unspecified time. A landfill has two major drawbacks, these drawbacks are leachate and the gases of decomposition. These impacts can be some what monitored and minimized by the
specifications listed under design. Siting and design of landfill operations to avoid disturbing water quality should be based on geological and hydrological considerations. The disposal options we have available to us are:
• Dumped in sand and gravel within open pits previously dug by bulldozer, pits then filled to control odor and other problems.
• Dumped on top of fill and mixed with refuse during compaction.
• Dewatered by the treatment plant, moved to landfill , dumped , and immediately buried.
• Only air-dried digested sludge accepted.
• City landfill disposal of sludge unregulated.
The most important factor of a landfill is to build it properly so that the environment is not disturbed in any fashion. There are several components to the design of a environmentally friendly landfill. These components are that the landfill should be placed on a compacted low permeable medium, preferably a clay layer. This layer is then covered by a impermeable membrane which is then covered by a granular substance to act as a secondary drainage system. Layers upon layers are built up, while each layer is separated by a granular membrane. This is done over and over again until the entire landfill is full. Then they cap off the landfill to prevent excess amounts of surface water from entering. The design of the landfill layers and the mound are:
• an above grade containment mound, sloped to support the weight of the waste and cover
• a liner system across the base to retard entry of water and subsequent percolation of leachate
• a leachate collection and removal system that is drained freely by gravity, with drainage above ground
The objectives of a properly designed landfill are to:
Protect groundwater quality
Protect air quality and conserve energy by installing a landfill gas recovery system
Minimize impact upon adjacent surface waters and wetlands
Utilize landfill space efficiently and extend site life
a cover system consisting of a layer with gas collection equipment, a composite liner, a drainage liner, and a permanent vegetative cover a monitoring system
Investigating a site is to first look at the type of soil and its bearing capacity. This should be done by digging boreholes at several designated locations over the entire landfill design site. There are several parameters which should be evaluated on the soil and they are :
A) unit weight of the soil
B) moisture content
C) void ratios
D) angle of internal friction
G) solution holding capacity
To determine the cost analysis of landfilling sludge you must evaluate the steps preceding it. After the sewage treatment plant has treated the sludge they send it to a dewatering site. This site reduces the sludge to 20% solid and 80% water. The actual cost of operating a dewatering facility is depends upon size and technology. This cost is not accessible, but after the dewatered sludge leaves the plant it is hauled by truck to the landfill site, which costs the sewage treatment plant approximately $91 a wet ton. Take into consideration that biosolids reduce the need for commercial fertilizers and can reduce fertilizer cost by over $100.00 an acre. The two contaminants of environmental concern from refuse disposal are gas and leachate. The leachate is generated because of the water the penetrates the landfill and the gas is due to the decomposing of the organic matter. Gas production from the organic matter begins before it is actually landfilled. the principal gases that are generated from the decomposing matter is carbon dioxide and methane. Carbon dioxide is important in the surrounding areas water quality, because it is soluble in water, unlike the other gases that can be produced within the landfill which are insoluble. When carbon dioxide is dissolved in water it lowers the pH , which creates a corrosive environment. It also creates an increase in water hardness.
Usually the effects of carbon dioxide are at a maximum during the first few months of decomposition and could continue on for a few years. As time goes on carbon dioxide values decrease and pose a lesser problem as the years go by. Leachate production within a landfill depends on the amount of water that enters the landfill. Leachate results when the amount of water entering exceeds the amount of water that can be retained by the waste. This is a major reason why site investigation and soil characteristics are so very essential in landfill design. The primary causes of excess water intrusion are due to a raise in groundwater elevation. Another consideration that should be evaluated is the topography and the climate of the area, because these two factors can cause a dramatic impact on the landfill if they are not assessed properly. The best approach to leachate management is to prevent or limit its production from the beginning. This is why proper design and elaborate research of an area are so very essential to a landfill and its operation.
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