PRE-STAGE BASICS
Before sludge undergoes treatment such as dewatering or thickening, it must be stored and pretreated. Sludge storage is an important, integral part of every wastewater sludge treatment and disposal system. Sludge storage provides many benefits including equalization of sludge flow to downstream processes, allowing sludge accumulation during times of non-operation of sludge-processing facilities, and allowing a uniform feed rate that enhances thickening, conditioning, and dewatering operations.
Sludge is stored within wastewater treatment process tankage, sludge treatment process systems, or separately in specially designed tanks. Sludge can be stored on a short-term or a long-term basis. Small treatment plants, where storage time may vary from several to 24 hours, may store sludge in wastewater clarification basins or sludge-thickening tanks. Larger plants often use aerobic digester, facultative lagoons, and other processes with long detention times to store sludge. The pretreatment of sludge is often necessary before dewatering or thickening can take place. It includes degritting and grinding. Sludge degritting involves the installation of grit removal and precessing facilities at the head works where raw wastewater first enters the treatment plant. As a result, there is reduced wear on influent pumping systems and primary sludge pumping, piping and thickening systems.
Sludge grinding involves shearing of large sludge solids into smaller particles. This method is used to prevent problems with operation of downstream processes. In- line grinders reduce cleaning and maintenance down time of equipment. The grinders can shear sludge solids to 6 to13 mm, depending on design requirements.
Sludge-pumping systems play an important part in wastewater treatment plants, particularly those operations experiencing average flows of greater than 1 million gallons per day (mgd). There are different types of pumps within this process.
Typical advantages of kinetic pumps for sludge transport include lower purchase cost, lower maintenance cost due to wear, less space used, and availability of both dry-well and submersible pumps. Advantages of positive displacement pumps include improved process control and pumping capability at high pressure and low flow.
Sludge cake storage (where a cake is the dewatered solid part of sludge) provides similar benefits for downstream disposal alternatives, like composting and incineration, to sludge storage which is used for thickening and dewatering. Storage of sludge cakes increases operational reliability, evens out flow fluctuations, and allows accumulation when downstream operations are not in service. Bins or hoppers are used to store sludge cakes. These can be made of any size form several cubic meters to 380 cubic meters capacity. Existing sludge dewatering operations can produce cakes that are 15 to 40% solids. These cakes range in consistency from
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pudding to damp cardboard. Since they will not flow by gravity in a pipe or channel, sludge cakes must be transported by one of the following methods:
mechanical conveyors such as flat or troughed belt, corrugated belt, or Archimedes screw; gravity drop from dewatering equipment into storage hoppers directly below; and pumping by positive displacement pumps.
Thermal Stabilization
Thermal stabilization is a heat process by which the bound water
(water associated with sludge) of the sludge solids is released by heating the sludge for short periods of time. Exposing the sludge to heat and pressure coagulates the solids, breaks down the cell structure, and reduces the hydration and hydrophilic (water loving) nature of the solids. The liquidportion of the sludge can then be separated from the solid by decanting and pressing.
Before any of the sludge can proceed to dewatering or thickening processes, it must be conditioned. Sludge conditioning involves chemical or thermal treatment to improve the efficiency of the downstream processes.
Chemical conditioning involves use of inorganic chemicals or organic polyelectrolytes, or both. The most commonly used inorganic chemicals are ferric chloride and lime. Other chemicals are popular outside of the U. S . . Organic polymers, introduced during the 1960's, are used for both sludge-thickening and dewatering processes. Their advantage over inorganics is that polymers don't greatly increase the amount of sludge production: 1 kg of inorganic chemicals added will produce 1 kg of extra sludge.
The disadvantage of polymers is their relatively high cost. There are several important factors that affect conditioning of sludge. They include: sludge characteristics, sludge handling, and sludge coagulation and flocculation. The fundamental purpose of sludge conditioning is to cause the aggregation of fine solids by coagulation with inorganic chemicals, flocculation with organic polymers, or both. A critical design parameter in conditioning is dosage. Selection of the right dosage of a chemical conditioner is critical for good performance. The dosage affects the solids content of sludge cakes as well as solids capture rate and solids disposal cost. Dosage is determined form pilot studies, bench tests, and on-line tests. In the following sections we will cover the basics of sludge stabilization and then conditioning. Our objective is to gain a working knowledge of these operations and to build our vocabulary.
CHEMICAL STABILIZATION
Chemical stabilization is a process whereby the sludge matrix is treated with
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chemicals in different ways to stabilize the sludge solids. Two common methods employed are lime stabilization, and the use of
chlorine.
The lime stabilization process can be used to treat raw primary, waste activated, septage and anaerobically digested sludge. The process involves mixing a large enough quantity of lime with the sludge to increase the pH of the mixture to 12 or more. This normally reduces bacterial hazards and odor to a negligible value, improves vacuum filter performance and provides satisfactory means of stabilizing the sludge prior to ultimate disposal.
Chemical Stabilization
Chemical stabilization is a process whereby the sludge matrix is treated with chemicals in different ways to stabilize the sludge solids.
Stabilization by chlorine addition has been developed and is marketed under the registered trade name "Purifax". The chemical conditioning of sludge with chlorine varies greatly from the more traditional methods of biological digestion or heat conditioning. First, the reaction is almost instantaneous. Second, there is very little volatile solids reduction in the sludge. There is some breakdown of organic material and formation of carbon dioxide and nitrogen; however, most of the conditioning is by the substitution or addition of chlorine to the organic compound to form new compounds that are biologically inert.
STABILIZATION VIA AEROBIC DIGESTION
Aerobic digestion is an extension of the activated sludge aeration process whereby waste primary and secondary sludge are continually aerated for long periods of time. In aerobic digestion the microorganisms extend into the endogenous respiration phase. This is a phase where materials previously stored by the cell are oxidized, with a reduction in the biologically degradable organic matter. This organic matter, from the sludge cells is oxidized to carbon dioxide, water and ammonia. The ammonia is further converted to nitrates as the digestion process proceeds. Eventually, the oxygen uptake rate levels off and the sludge matter is reduced to inorganic matter and relatively stable volatile solids.
The primary advantage of aerobic digestion is that it produces a biologically stable end product suitable for subsequent treatment in a variety of processes. Volatile solids reductions similar to anaerobic digestion are possible. Some parameters affecting the aerobic digestion process are:
1.
2. sludge temperature,
3. system oxygen requirements, 4. sludge loading rate,
The rate of sludge oxidation,
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5 . sludge age, and
6 . sludge solids characteristics.
Aerobic digestion has been applied mostly to various forms of activated sludge treatment, usually "total oxidation" or contact stabilization plants. However, aerobic digestion is suitable for many types of municipal and industrial wastewater sludge, including trickling filter humus as well as waste activated sludge. Any design for an aerobic digestion system should include: an estimate of the quantity of sludge to be produced, the oxygen requirements, the unit detention time, the efficiency desired, and the solids loading rate. Aerobic digestion tanks are normally not covered or heated, therefore, they are much cheaper to construct than covered, insulated, and heated anaerobic digestion tanks. In fact, an aerobic digestion tank can be considered to be a large open aeration tank. Similar to conventional aeration tanks, the aerobic digesters may be designed for spiral roll or cross roll aeration using diffused air equipment. The system should have sufficient flexibility to allow sludge thickening by providing supernatant decanting facilities. The advantages most often claimed for aerobic digestion are:
A humus-like, biologically stable end product is produced.
The stable end product has no odors, therefore, simple land disposal, such as lagoons, is feasible.
Capital costs for an aerobic system are low, when compared with anaerobic digestion and other schemes.
Aerobically digested sludge usually has good dewatering characteristics. When applied to sand drying beds, it drains well and redries quickly if rained upon.
The volatile solids reduction can be equal to those achieved by anaerobic digestion.
Supernatant liquors from aerobic digestion have a lower BOD than those from anaerobic digestion. Most tests indicated that BOD would be less than 100 ppm.
This advantage is important because the efficiency of many treatment plants is reduced as a result of recycling high BOD supernatant liquors. There are fewer operational problems with aerobic digestion than with the more complex anaerobic form because the system is more stable. As a result, less skillful and costly labor can be used to operate the facility. In comparison with anaerobic digestion, more of the sludge basic fertilizer values are recovered.
The major disadvantage associated with aerobic digestion is high power costs. This factor is responsible for the high operating costs in comparison with anaerobic digestion. At small waste treatment plants, the power costs may not be significant but they certainly would be at large plants. Aerobically digested sludge does not always settle well in subsequent thickening processes. This situation leads to a thickening tank decant having a high solids concentration. Some sludge do not dewater easily by vacuum filtration after being digested aerobically. Two other minor disadvantages are the lack of methane gas production and the variable solids
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reduction efficiency with varying temperature changes.
In a typical plant operation the pollutants dissolved in the wastewater or that would not settle in the primary clarifiers flow on in the wastewater to the Secondary treatment process. Secondary treatment further reduces organic matter (BOD,) through the addition of oxygen to the wastewater which provides an aerobic environment for microorganisms to biologically break down this remaining organic matter.
This process increases the percent removals of BOD and TSS to a minimum of 85 percent. A secondary treatment facility can be comprised of Oxygenation Tanks, Pure Oxygen Generating Plant, Liquid Oxygen Storage Tanks, Secondary Clarifiers, Return Sludge Pumping Station and
Air Reauiremenk 15 - 20 cfm per 1,000 cubic feet of digester capacity is adequate. The air supplied must keep the solids in suspension; this requirement may exceed the sludge oxidation requirement. A dissolved oxygen concentration of I to 2 ppm should be maintained in the aerobic digestion tanks.
Detention Time: Waste activated sludge only, afler sludge thickening.
IO - 15 days volumetric displacement time. If sludge temperatures are much less than 60°F, more capacity should be provided. Primary sludge mixed with waste activated or trickling filter humus. 20 days displacement time in moderate climates.
Splitter Box, Sludge Thickeners and Pumping Station, Sludge Dewatering Building Addition and modifications to the existing Service Water Pumping Station. The Pure Oxygen Generation System often incorporates a pressure swing adsorption (PSA) system oxygen generating system A PSA system will provide a certain amount (as tons per day) of pure oxygen to the oxygenation system. As backup to the oxygen generating system, spare oxygen storage tanks containing liquid oxygen can be included in the design. Figure 5 illustrates what an aeration reactor looks like.
The oxygenation system is comprised of several covered oxygenation tanks, mechanical mixing system, and pressure-controlled oxygen feed and oxygen purity- controlled venting system. The primary effluent enters the head end of the tanks where it mixes with return activated sludge which consists of microorganisms
Figure 5. Example of aeration reactors.
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"activated" by the organic matter and oxygen. This combination of primary effluent and return sludge forms a mixture known as "Mixed Liquor". This mixed liquor is continuously and thoroughly mixed by the mechanical mixer in each tank. The oxygen gas produced in the PSA system is introduced into the first stage of each tank and then remains in contact with the mixed liquor throughout the oxygenation system. Secondary clarifiers like the one illustrated in Figure 6 are used in this process.
Figure 6. Diagram of a secondary clarifier.
Once the mixed liquor goes through the complete oxygenation process, it flows to four secondary clarifiers where the biological solids produced during the oxygenation process are allowed to settle and be pumped back to the head of the system. These settled solids being pumped, called return activated sludge, mix with the primary effluent to become mixed liquor. Since the population of microorganisms is growing some microorganisms in the return activated sludge are removed from the system. This solids waste stream is called waste activated sludge ( WAS ) and flows to the secondary gravity thickener for solids processing . The
cleaned wastewater flows over the weir of the secondary clarifier and on to the disinfection ( chlorination )-process. The activated sludge process describes is an aerobic, suspended growth, biological treatment method. It employs the metabolic reactions of microorganisms to produce a high quality effluent by oxidation and conversion of organics to carbon dioxide, water and biosolids (sludge). Basically the system speeds up nature and supplies oxygen so the aquatic environment will not have to. High concentrations of microorganisms ( compared to a natural aquatic environment ) in the activated sludge use the pollutants in the primary treated wastewater as food and remove the dissolved and non-settleable pollutants from the wastewater. These pollutants are incorporated into the microorganisms bodies and will then settle in the secondary clarifiers. Oxygen needs to be supplied for the microorganisms to survive and consume the pollutants.
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STABILIZATION VIA ANAEROBIC DIGESTION
The purpose of digestion is to attain both of the objectives of sludge treatment -- a reduction in volume and the decomposition of highly putrescible organic matter to relatively stable or inert organic and inorganic compounds. Additionally, anaerobic sludge digestion produces a valuable by-product in the form of methane gas (the primary constituent of natural gas, which we can bum for heat or convert to electricity). Sludge digestion is carried out in the absence of free oxygen by anaerobic organisms. It is, therefore, anaerobic decomposition. The solid matter in raw sludge is about 70 percent organic and 30 percent inorganic or mineral.
Much of the water in wastewater sludge is "bound" water which will not separate from the sludge solids. The facultative and anaerobic organisms break down the complex molecular structure of these solids setting free the "bound" water and obtaining oxygen and food for their growth.
Anaerobic digestion involves many complex biochemical reactions and depends on many interrelated physical and chemical factors. For purposes of simplification, the anaerobic degradation of domestic sludge occurs in two steps. In the first step, acid forming bacteria attack the soluble or dissolved solids, such as the sugars. From these reactions organic acids, at times up to
several thousand ppm, and gases, such as carbon dioxide and hydrogen sulfide are formed. This is known as the stage of acid fermentation and proceeds rapidly. It is followed by a period of acid digestion in which the organic acids and nitrogenous compounds are attacked and liquefied at a much slower rate.
In the second stage of digestion, known as the period of intensive digestion, stabilization and gasification, the more resistant nitrogenous materials, such as the proteins, amino-acids and others, are attacked. The pH value must be maintained from 6.8 to 7.4. Large volumes of gases with a 65 or higher percentage of methane are produced. The organisms which
Hydrogen SuljXe or H,S smells like rotten eggs. It is highly dangerous at airborne concentrations greater than 50 ppm. Exposure to H2S above lOppm forprolonged periods will cause olefactary saturation - i.e., you will not be able to smell the
characteristic rotten egg odor and believe you are not being exposed to an inhalation
convert organic acids to methane and carbon dioxide gases are called methane formers. The solids remaining are relatively stable or only slowly putrescible, can be disposed of without creating objectionable conditions and have value in agriculture.
The whole process of sludge digestion may be likened to a factory production line where one group of workers takes the raw material and conditions it for a second group with different "skills" who convert the material to the end products.
In a healthy, well operating digester, both of the above stages are taking place
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continuously and at the same time. Fresh wastewater solids are being added at frequent intervals with the stabilized solids being removed for further treatment or disposal at less frequent intervals. The Supernatant digester liquor, the product of liquefaction and mechanical separation is removed frequently to make room for the added fresh solids and the gas is, of course, being removed continuously.
While all stages of digestion may be proceeding in a tank at the same time with the acids produced in the first stage being neutralized by the ammonia produced in subsequent stages, best and quickest results are obtained when the over-all pH of 6.8 to 7.4 predominates. The first stage of acid formation should be evident only in starting up digestion units. Once good alkaline digestion is established, the acid stage is not apparent unless the normal digestion becomes upset by overloading, poisonous chemicals or for other reasons. It is critical to the overall process to maintain balanced populations of acid formers and methane formers. The methane formers are more sensitive to environmental conditions and slower growing than the acid forming group of bacteria and control the overall reactions.
The progress of digestion can be measured by the destruction of organic matter (volatile solids), by the volume and composition of gases produced, by the pH, volatile acids, and alkalinity concentration. It is recommended that no on parameter or test be used to predict problems or control digesters. Several of the following parameters must be considered together.
The reduction of organic matter as measured by the volatile solids indicates the completeness of digestion. Raw sludge usually contains from 60 to 70 percent volatile solids while a well hgested sludge may have as little as 50 percent. This would represent a volatile solids reduction of about 50 percent. Volatile solids reduction should be measured weekly and trended. Downward trends in volatile solids reduction might mean:
0 Temperature too low and/or poor temperature control.
Ineffective mixing of digester contents.
Grit and/or scum accumulations are excessive.
Low volatile solids in raw sludge feed.
0 Digester is overloaded.
0 0 0
A well digested sludge should be black in color, have a not unpleasant tarry odor and, when collected in a glass cylinder, should appear granular in structure and show definite channels caused by water rising to the top as the solids settle to the bottom.
For domestic wastewater in a normally operating digestion tank, gas production should be in the vicinity of 12 cu.ft. of gas per day per Lbs of volatile matter destroyed. This would indicate that for a 50 percent reduction of volatile matter, a gas yield of six cu.ft. per Lbs of volatile matter added should be attained. The quantity of gases produced should be relatively constant if the feed rate is constant.