How is Wastewater Sludge Treated for Disposal?

SUEZ provides high-performance industrial sludge treatment solutions for any size population and virtually any influent condition. SUEZ’ team is involved in every stage of a project, from process design to equipment supply to operations training.

As a complete treatment line provider of sludge treatment processes, SUEZ offers integrated solutions for industry, including advanced anaerobic digestion (AD), and biogas treatment products, dewatering and drying, thermal oxidation of waste and sludge streams.

Careful evaluation of each application ensures customers receive the most efficient combination of technology and products at the lowest overall cost for industrial sludge treatment and disposal. SUEZ’ design engineers review your entire treatment process to find ways to improve water supply and control pollution economically.
What is Wastewater Sludge?

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.

Industrial Sludge Treatment Process
Wastewater treatment is a concentration process in which waterborne contaminants are removed from the larger wastewater stream and concentrated in a smaller side stream of slurry, called sludge. The side stream is too large to be disposed of directly, so further concentration processes are required. These processes are called “solid waste handling” operations.

Sludge Thickening & Dewatering Processes

SUEZ has developed a number of different sludge thickening and dewatering processes. We recommend a technique according to the characteristics of the sludge and the objectives of the treatment process. Two processes can be used to thicken sludge: static thickening, by gravitational settling, and dynamic thickening, a concentration process that calls on dynamic energy.

After the thickening phase, dewatering uses different techniques that are selected to best meet the operating conditions of the process chosen by SUEZ. The increasing volume of sludge produced by water treatment, in line with population growth and urban sprawl, is a major cause for concern for local authorities and operators. Treating sludge to reduce volume and to transform it into recoverable products has become a necessity.

SUEZ helps you to define and choose the optimal sludge treatment solution for you according to the origin and composition of the sludge and treatment by-products the final destination of the sludge (recycling, recovery, disposal).

Process 1: Stabilization/Digestion

Sludge stabilization is a treatment technique applied to biological sludge to reduce its odor-causing or toxic properties. This treatment often reduces the amount of solids as a side effect. Anaerobic and aerobic digestion, lime treatment, chlorine oxidation, heat treatment, and composting fall into this category.

Process 2: Anaerobic Digestion

Anaerobic digestion takes place in an enclosed tank. Sludge solids are decreased due to the conversion of biomass to methane and carbon dioxide. The methane can be recovered for its heating value.

Process 3: Aerobic Digestion

Aerobic digestion is the separate aeration of sludge in an open tank. Oxidation of biodegradable matter, including cell mass, occurs under these conditions. As in anaerobic digestion, there is a decrease in sludge solids, and the sludge is well stabilized with respect to odor formation. Capital costs are lower than those of anaerobic digestion, but operating costs are higher, and there is no by-product methane production.

Process 4: Lime Treatment

Stabilization by lime treatment does not result in a reduction of organic matter. Addition of sufficient lime to maintain the pH of the sludge above 11.0 for 1-14 days is considered sufficient to destroy most bacteria.

Process 5: Sludge Conditioning

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.

Dewatering Process

Process 1: Belt Filter Press

Belt filter presses are designed on the basis of a very simple concept. Sludge is sandwiched between two tensioned porous belts and passed over and under rollers of various diameters. At a constant belt tension, rollers of decreasing diameters exert increasing pressure on the sludge, thus squeezing out water. Although many different designs of belt filter presses are available, they all incorporate a polymer conditioning unit, a gravity drainage zone, a compression (low-pressure) zone, and a shear (high-pressure) zone.

Process 2: Polymer Conditioning Unit

Polymer conditioning can take place in a small tank, in a rotating drum attached to the top of the press, or in the sludge line. Usually, the press manufacturer supplies a polymer conditioning unit with the belt filter press.

Process 3: Gravity Drainage Zone

The gravity drainage zone is a flat or slightly inclined belt, which is unique to each press model. In this section, sludge is dewatered by the gravity drainage of free water. The gravity drainage zone should increase the solids concentration of the sludge by 5 percent to 10 percent. If the sludge does not drain well in this zone, the sludge can squeeze out from between the belts or the belt mesh can become blinded. The effectiveness of the gravity drainage zone is a function of sludge type, quality, and conditioning, along with the screen mesh and the design of the drainage zone.

Process 4: Centrifuges

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.

Process 5: Plate and Frame Press

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.

Process 6: Sludge Drying Beds

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.

Wastewater Sludge Disposal
Disposal of the sludge generated by wastewater treatment plants is dependent on government regulations (such as the Resource Conservation and Recovery Act), geographical location, and sludge characteristics, among other things. Final wastewater sludge disposal methods include reclamation, incineration, land application, and landfill.

Process 1: Reclamation of Sludge

Because of costs associated with the disposal of wastewater sludge, each waste stream should be evaluated for its reclamation potential. Energy value, mineral content, raw material makeup, and by-product markets for each sludge should be evaluated. Examples include burning of digester gas to run compressors, recalcination of lime sludge to recover calcium oxide, return of steel mill thickener sludge to the sinter plant, and marketing of by-product metallic salts for wastewater treatment use.

Process 2: Sludge Incineration

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.

Process 3: Land Application

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.

Process 4: Landfill

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.

Biosolids / Biogas
Designed to recycle biosolids into methane and valuable byproducts, SUEZ’ advanced anaerobic digestion technologies use bacteria in the absence of oxygen to break down industrial sludge and other matter to create biogas. The biogas can be combusted or oxidized and used for heating or with a CHP engine to produce electricity and heat; it can also be compressed and used as fuel for vehicles with the cake produced, used as a fertilizer.
Discover SUEZ' Wastewater Treatment Benefits

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.