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 Us 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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