Sludge is the residual slurry that is produced during sewage treatment of industrial or municipal wastewater. According to the Water Environment Association, wastewater typically averages 99.94 percent water by weight; only a small 0.06 percent is actually waste material, from which sludge is derived.
Aside from the most obvious human waste, our daily activities contribute many other water pollutants, including food particles, paper products, dirt, oil and grease, proteins, organic materials such as sugars, inorganic materials such as salts, personal care products, pharmaceuticals, cleaning chemicals, and hundreds of other chemicals.
The purpose of sludge conditioning is to provide a rigid sludge structure of a porosity and pore size sufficient to allow drainage. Biological sludges are conditioned with iron chloride, lime, and synthetic cationic polymers, either separately or in combination. Heat conditioning and low-pressure oxidation are also used for biological sludges. Inorganic sludges are conditioned with iron chloride,, lime, and either cationic or anionic polymers.
Typically, sludge from a final liquid-solids separation unit may contain from 1 percent to 5 percent total suspended solids. Because of the cost savings associated with handling smaller volumes of sludge, there is an economic incentive to remove additional water. Dewatering equipment is designed to remove water in a much shorter time span than nature would by gravity. Usually, an energy gradient is used to promote rapid drainage. This requires frequent conditioning of the sludge prior to the dewatering step.
Conditioning is necessary due to the nature of the sludge particles. Both inorganic and organic sludge consist of very small (colloidal), intermediate, and large particles. The large particles, or flocs, are usually compressible. Under an energy gradient, these large flocs compress and prevent water from escaping. The pressure drop through the sludge cake, due to the decrease in porosity and pore sizing, exceeds available energy, and dewatering ceases.
Centrifugal force, 3,500 to 6,000 times the force of gravity, is used to increase the sedimentation rate of solid sludge particles. The most common centrifuge found in waste treatment dewatering applications is the continuous bowl centrifuge. The two principal elements of a continuous solid bowl centrifuge are the rotating bowl and the inner screw conveyor. The bowl acts as a settling vessel; the solids settle due to centrifugal force from its rotating motion.
The screw conveyor picks up the solids and conveys them to the discharge port. Often, operation of centrifugal dewatering equipment is a compromise between quality, cake dryness, and sludge throughput. For example, an increase in solids throughput reduces clarification capacity, causing a decrease in solids capture. At the same time, the cake is drier due to the elimination of fine particles that become entrained in the. The addition of polymers, with their ability to agglomerate fine particles, can result in increased production rates without a loss in quality. Polymers are usually fed inside the bowl because shear forces may destroy flocs if they are formed prior to entry. Also, large particles settle rapidly in the first stage of the bowl. Thus, economical solids recovery can be achieved through internal feeding of polymers after the large particles have settled.
A plate and frame filter press is a batch operation consisting of vertical plates held in a frame. A filter cloth is mounted on both sides of each plate. Sludge pumped into the unit is subjected to pressure as the plates are pressed together. As the sludge fills the chamber between individual plates, the filtrate flow ceases, and the dewatering cycle is completed. This cycle usually lasts up to two hours.
Because of the high pressures, blinding of the filter cloth by small sludge particles can occur. A filter precoat (e.g., diatomaceous earth) can be used to prevent filter blinding. Proper chemical conditioning of the sludge reduces or eliminates the need for precoat materials. At greater pressures, the effectiveness of synthetic polymers is reduced; therefore, inorganic chemicals, such as ferric chloride and lime, are often used instead of polymers.
Sludge drying beds consist of a layer of sand over a gravel bed. Underdrains spaced throughout the system collect the filtrate, which usually is returned to the wastewater plant.
Water is drained from the sludge cake by gravity through the sand and gravel bed. This process is complete within the first two days. All additional drying occurs by evaporation, which takes from two to six weeks. For this reason, climatic conditions, such as frequency and rate of precipitation, wind velocity, temperature, and relative humidity, play an important role in the operation of sludge drying beds. Often, these beds are enclosed to aid in dewatering. Chemical conditioning also reduces the time necessary to achieve the desired cake solids.
Biological sludge can be disposed of by incineration; the carbon, nitrogen, and sulfur are removed as gaseous by-products, and the inorganic portion is removed as ash. Old landfill sites are filling up and new ones are becoming increasingly difficult to obtain. Therefore, waste reduction through incineration is becoming a favored disposal practice.
Several combustion methods are available, including hogged fuel boilers, wet air oxidation and kiln, multiple hearth furnace, and fluidized bed combustion processes.
Sludge incineration is a two-step process involving drying and combustion. Incineration of waste sludge usually requires auxiliary fuel to maintain temperature and evaporate the water contained in the sludge. It is critically important to maintain a low and relatively constant sludge moisture.
Sludge produced from biological oxidation of industrial wastes can be used for land application as a fertilizer or soil conditioner. A detailed analysis of the sludge is important in order to evaluate toxic compound and heavy metal content, leachate quality, and nitrogen concentration.
Soil, geology, and climate characteristics are all important considerations in determining the suitability of land application, along with the type of crops to be grown on the sludge-amended soil. Sludge application rates vary according to all of these factors.
Landfill is the most common method of industrial wastewater treatment plant sludge disposal.
Care must be taken to avoid pollution of groundwater. The movement and consequent recharge of groundwater is a slow process, so contamination that would be very small for a stream or river can result in irreversible long-term pollution of the groundwater. Many states require impermeable liners in landfill disposal sites. This requirement limits liners to a few natural clays and commercial plastic liners. In addition to impermeable liners, leachate collection and treatment systems are typically required for new and remediated landfills.
Steps can be taken to reduce leachate and leachate contamination. Decreasing the moisture in the sludge removes water that would eventually be available as leachate. Proper consideration of the hydraulics of the landfill site can capture more rainfall as runoff and eliminate ponding and its contribution to leachate.
Wastewater treatment ensures water quality standards are met before treated water is discharged back in the environment. Industrial wastewater systems are facing growing challenges due to aging infrastructure, a growing number of users, and additional water quality measures.
SUEZ provides effective solutions to rehabilitate and maintain wastewater infrastructure in fit for purpose condition to meet applicable regulations and protect public health and wildlife. Our sustainable solutions help preserve your large capital investments. We also offer innovative technology to help industrial treatment plants rethink biosolid disposal through nutrient recovery programs.
Contact a representative to learn more about SUEZ' wastewater sludge treatment services.