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Introduction to sludge management. 3. Main topic. Items covered. Sewage sludge: characteristics and production. • Sludge production in wastewater treatment.
Table of contents
At the same time, sewage sludge from wastewater treatment plants contains nutrients, micronutrients and particles that increase humus in the soil. It has remarkable potential for energy production, particularly by producing biogas or direct incineration. Proper and environmentally sustainable management before final disposal is required.
The most common treatment options include anaerobic digestion , aerobic digestion, and composting. Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. Though allowing shorter retention time and thus smaller tanks , thermophilic digestion is more expensive in terms of energy consumption for heating the sludge. Mesophilic anaerobic digestion MAD is also a common method for treating sludge produced at sewage treatment plants. The sludge is fed into large tanks and held for a minimum of 12 days to allow the digestion process to perform the four stages necessary to digest the sludge.
These are hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In this process the complex proteins and sugars are broken down to form more simple compounds such as water, carbon dioxide, and methane. Anaerobic digestion generates biogas with a high proportion of methane that may be used to both heat the tank and run engines or microturbines for other on-site processes. Methane generation is a key advantage of the anaerobic process. Its key disadvantage is the long time required for the process up to 30 days and the high capital cost.
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Sufficient energy can be generated in this way to produce more electricity than the machines require. Aerobic digestion is a bacterial process occurring in the presence of oxygen resembling a continuation of the activated sludge process. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. Once there is a lack of organic matter, bacteria die and are used as food by other bacteria.
This stage of the process is known as endogenous respiration. Solids reduction occurs in this phase.
Because the aerobic digestion occurs much faster than anaerobic digestion, the capital costs of aerobic digestion are lower. However, the operating costs are characteristically much greater for aerobic digestion because of energy used by the blowers, pumps and motors needed to add oxygen to the process. However, recent technological advances include non-electric aerated filter systems that use natural air currents for the aeration instead of electrically operated machinery. Aerobic digestion can also be achieved by using diffuser systems or jet aerators to oxidize the sludge.
Fine bubble diffusers are typically the more cost-efficient diffusion method, however, plugging is typically a problem due to sediment settling into the smaller air holes. Coarse bubble diffusers are more commonly used in activated sludge tanks or in the flocculation stages. A key component for selecting diffuser type is to ensure it will produce the required oxygen transfer rate. Composting is an aerobic process of mixing sewage sludge with agricultural byproduct sources of carbon such as sawdust, straw or wood chips.
In the presence of oxygen, bacteria digesting both the sewage sludge and the plant material generate heat to kill disease-causing microorganisms and parasites. Stiff materials like corn cobs, nut shells, shredded tree-pruning waste, or bark from lumber or paper mills better separate sludge for ventilation than softer leaves and lawn clippings. Uniform distribution of pathogen-killing temperatures may be aided by placing an insulating blanket of previously composted sludge over aerated composting piles.
Initial moisture content of the composting mixture should be about 50 percent; but temperatures may be inadequate for pathogen reduction where wet sludge or precipitation raises compost moisture content above 60 percent. Composting mixtures may be piled on concrete pads with built-in air ducts to be covered by a layer of unmixed bulking agents.
Odors may be minimized by using an aerating blower drawing vacuum through the composting pile via the underlying ducts and exhausting through a filtering pile of previously composted sludge to be replaced when moisture content reaches 70 percent. Liquid accumulating in the underdrain ducting may be returned to the sewage treatment plant; and composting pads may be roofed to provide better moisture content control.
After a composting interval sufficient for pathogen reduction, composted piles may be screened to recover undigested bulking agents for re-use; and composted solids passing through the screen may be used as a soil amendment material with similar benefits to peat. The optimum initial carbon-to-nitrogen ratio of a composting mixture is between ; but the composting ratio of agricultural byproducts may be determined by the amount required to dilute concentrations of toxic chemicals in the sludge to acceptable levels for the intended compost use. Incineration of sludge is less common because of air emissions concerns and the supplemental fuel typically natural gas or fuel oil required to burn the low calorific value sludge and vaporize residual water.
Co-firing in municipal waste-to-energy plants is occasionally done, this option being less expensive assuming the facilities already exist for solid waste and there is no need for auxiliary fuel.
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Simple sludge drying beds are used in many countries, particularly in developing countries, as they are a cheap and simple method to dry sewage sludge. Drainage water must be captured; drying beds are sometimes covered but usually left uncovered. Mechanical devices to turn over the sludge in the initial stages of the drying process are also available on the market.
Drying beds are typically composed of four layers consisting of gravel and sand. The first layer is coarse gravel that is 15 to 20 centimeters thick. Followed by fine gravel that is 10 centimeters thick. The third layer is sand that can be between 10 and 15 centimeters and serves as the filter between the sludge and gravel.
Sludge dries up and water percolates to the first layer that is collected at the drainage pipe that is beneath all layers.
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When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Processes for reducing water content include lagooning in drying beds to produce a cake that can be applied to land or incinerated; pressing , where sludge is mechanically filtered, often through cloth screens to produce a firm cake; and centrifugation where the sludge is thickened by centrifugally separating the solid and liquid.
Sludges can be disposed of by liquid injection to land or by disposal in a landfill.
3. Wastewater treatment
The same values are valid for maximum permissible annual rate of PTE. Source: Department of the Environment Research carried out in the U. Carlton-Smith, has shown that the amounts of Cd, Ni, Cu, Zn and Pb applied in liquid sludge at three experimental sites could be accounted for by soil profile analyses five years after sludge applications, with the exception of Cu and Zn applied to a calcareous loam soil.
These field experiments also determined the extent of transfer of metals from sludge-treated soil into the leaves and edible parts of six crops of major importance to UK agriculture and the effect of metals on yields of these crops. Although all the plots received sufficient inorganic fertilizer to meet crop requirements for nutrients, the applications of sludge had some effects on crop yields.
However, this yield reduction was not thought to be due to metals but the most likely explanation was lodging of the crop as a result of excessive nitrogen in the soil. Increases in metal concentrations in the soil due to sludge applications produced significant increases in Cd, Ni, Cu and Zn concentrations in the edible portion of most of the crops grown: wheat, potato, lettuce, red beet, cabbage and ryegrass.
In most cases there was no significant increase of Pb in crop tissue in relation to Pb in the soil from sludge application, suggesting that lead is relatively unavailable to crops from the soil. The availability of metals to crops was found to be lower in soil treated with bed-dried sludge cake compared with liquid sludge, the extent being dependent on the crop.
Even though the Ni, Cu and Zn concentrations in the soils treated with high rates of application of liquid and bed-dried sludges were close to the maximum levels set out in the EC Directive and the zinc equivalent of sludge addition exceeded the maximum permitted in U. This was in lettuce grown on clay soil, when Cu and Zn levels exceeded upper critical concentrations at high rates of sludge application.
Sludge stabilization
Sludge must not be applied to growing soft fruit or vegetable crops nor used where crops are grown under permanent glass or plastic structures Department of the Environment, The EC Directive Council of the European Communities, requires a mandatory 3-week no grazing period for treated sludge applied to grassland but prohibits the spreading of untreated sludge on grassland unless injected. Treated sludge can be applied to growing cereal crops without constraint but should not be applied to growing turf within 3 months of harvesting or to fruit trees within 10 months of harvesting.
When treated sludge is applied before planting such crops as cereals, grass, fodder, sugar beet, fruit trees, etc. In general, untreated sludge should only be cultivated or injected into the soil before planting crops but can be injected into growing grass or turf, with the constraints on minimum time to harvesting as already mentioned. The sludge must not contain non-degradable materials, such as plastics, which would make land disposal unsightly. Movement of sludge by tanker from sewage treatment plant to agricultural land can create traffic problems and give rise to noise and odour nuisance.
Vehicles should be carefully selected for their local suitability and routes chosen so as to minimize inconvenience to the public. Access to fields should be selected after consultation with the highway authority and special care must be taken to prevent vehicles carrying mud onto the highway. Odour control is the most important environmental dimension of sludge application to land.