Many industries use large volumes of water in their
manufacturing operations. Because some of this water becomes contaminated, it
requires treatment before discharge (Figure 37-1).
in determining the effects of industrial waste discharges have led to the
adoption of stringent environmental laws, which define the degree of treatment
necessary to protect water quality. Discharge permits, issued under the National
Pollutant Discharge Elimination System (NPDES), regulate the amount of
pollutants that an industry can return to the water source. The permitted
quantities are designed to ensure that other users of the water will have a
source that meets their needs, whether these needs are for municipal water
supply, industrial or agricultural uses, or fishing and recreation.
Consideration is given to the feasibility of removing a pollutant, as well as
the natural assimilative capacity of the receiving stream. This assimilative
capacity varies with the type and amount of pollutant.
treatment plants are designed to convert liquid wastes into an acceptable final
effluent and to dispose of solids removed or generated during the process. In
most cases, treatment is required for both suspended and dissolved contaminants.
Special processes are required for the removal of certain pollutants, such as
phosphorus or heavy metals.
can be recycled for reuse in plant processes to reduce disposal requirements
(Figure 37-2). This practice also reduces water consumption.
amount of organic material that can be discharged safely is defined by the
effect of the material on the dissolved oxygen level in the water. Organisms in
the water use the organic matter as a food source. In a biochemical reaction,
dissolved oxygen is consumed as the end products of water and carbon dioxide are
formed. Atmospheric oxygen can replenish the dissolved oxygen supply, but only
at a slow rate. When the organic load causes oxygen consumption to exceed this
resupply, the dissolved oxygen level drops, leading to the death of fish and
other aquatic life. Under extreme conditions, when the dissolved oxygen
concentration reaches zero, the water may turn black and produce foul odors,
such as the "rotten egg" smell of hydrogen sulfide. Organic compounds
are normally measured as chemical oxygen demand (COD) or biochemical oxygen
and phosphorus are essential to the growth of plants and other organisms.
However, nitrogen compounds can have the same effect on a water source as
carbon-containing organic compounds. Certain organisms use nitrogen as a food
source and consume oxygen.
is a concern because of algae blooms that occur in surface waters due to its
presence. During the day, algae produce oxygen through photosynthesis, but at
night they consume oxygen.
discharged with a waste stream may settle immediately at the discharge point or
may remain suspended in the water. Settled solids cover the bottom-dwelling
organisms, causing disruptions in population and building a reservoir of
oxygen-consuming materials. Suspended solids increase the turbidity of the
water, thereby inhibiting light transmittance. Deprived of a light source,
photosynthetic organisms die. Some solids can coat fish gills and cause
Acids and Alkalies
natural buffering system of a water source is exhausted by the discharge of
acids and alkalies. Aquatic life is affected by the wide swings in pH as well as
the destruction of bicarbonate alkalinity levels.
metals are toxic and affect industrial, agricultural, and municipal users of the
water source. Metals can cause product quality problems for industrial users.
Large quantities of discharged salts necessitate expensive removal by downstream
industries using the receiving stream for boiler makeup water.
REMOVAL OF INSOLUBLE CONTAMINANTS
physical methods may be used for the removal of wastewater contaminants that are
insoluble in water, such as suspended solids, oil, and grease. Ordinarily,
water-soluble contaminants are chemically converted to an insoluble form to
allow removal by physical methods. Essentially, biological waste treatment is
this conversion of soluble contaminants to insoluble forms.
waste treatment systems employ a gravity separation step for suspended particle
or oil removal.
settling rate of a particle is defined in terms of "free" versus
"hindered" settling. A free settling particle's motion is not affected
by that of other particles, the vessel's walls, or turbulent currents. A
particle has a hindered settling rate if there is any interference from these
The free settling of a discrete particle in a rising
fluid can be described as the resolution of several forces-gravity, the drag exerted on the particle, and the buoyant force
as described by Archimedes' principle. The particle's velocity increases until
it reaches a terminal velocity as determined by these forces. The terminal
velocity is then:
gravitation constant, ft/sec2
mass of the particle, lb
density of the particle, lb/ft3
density of the fluid, lb/ft3
cross sectional area of the particle ex-posed to the direction of motion,
drag coefficient, a function of particle geometry
settling is employed primarily for removal of inorganic suspended solids, such
as grit and sand. Therefore, in the approximation of the drag coefficient, it is
assumed that particles are spherical. Further, if a Reynolds number of less than
2.0 is assumed, the settling velocity of a discrete particle can be described by
Stokes' settling equation:
particle diameter, ft
fluid viscosity, lb/ft-sec
terminal velocity of a particle in the "free" settling zone is a
function of its diameter, the density difference between the particle and the
fluid, and the fluid viscosity.
The equipment employed for gravity separation for waste
treatment is normally either a rectangular basin with moving bottom scrapers for
solids removal or a circular tank with a rotating bottom scraper. Rectangular
tanks are normally sized to decrease horizontal fluid velocity to approximately
1 ft/min. Their lengths are three to five times their width, and their depths
are 3-8 ft.
clarifiers (see Figure 37-3) are ordinarily sized according to surface area,
because velocity must be reduced below the design particle's terminal velocity.
The typical design provides a rise rate of 600-800 gpd/ft2.
wastewater contains appreciable amounts of hydrocarbons, removal of these
contaminants becomes a problem. Oil is commonly lower in density than water;
therefore, if it is not emulsified, it can be floated in a separate removal
stage or in a dual-purpose vessel that allows sedimentation of solids. For
example, the refining industry uses a rectangular clarifier with a surface
skimmer for oil and a bottom rake for solids as standard equipment. This design,
specified by the American Petroleum Institute, is designated as an API
separator. The basic principles govern-ing the separation of oil from water by
gravity differential are also expressed by Stokes' Law.
the density differential is not sufficient to separate oil and oil-wetted
solids, air flotation may be used to enhance oil removal. In this method, air
bubbles are attached to the contaminant particles, and thus the apparent density
difference between the particles is increased.
air flotation (DAF) is a method of introducing air to a side stream or recycle
stream at elevated pressures in order to create a supersaturated stream. When
this stream is introduced into the waste stream, the pressure is reduced to
atmospheric, and the air is released as small bubbles. These bubbles attach to
contaminants in the waste, decreasing their effective density and aiding in
most important operational parameters for contaminant removal by dissolved air
recycle or slip stream flow rate
influent total suspended solids (TSS) including oil and grease
pressure, recycle, and influent TSS are normally related in an air-to-solids
(A/S) ratio expressed as:
a constant, approximately 1.3
the solubility of air at standard conditions, mL/L
air dissolved/Sa, usually 0.5-0.8
operating pressure, atm
recycle rate, gpm
influent suspended solids, mg/L
wastewater flow, gpm
A/S ratio is most important in determining effluent TSS. Recycle flow and
pressure can be varied to maintain an optimal A/S ratio. Typical values are
a DAF system, the supersaturated stream may be the entire influent, a slip
stream, fresh water, or a recycle stream. Recycle streams are most common,
because pressurization of a high- solids stream through a pump stabilizes and
disperses oil and oil-wetted solids.
in gravity settling, air flotation units are designed for a surface loading rate
that is a function of the waste flow and rise velocity of the contaminants
floated by air bubbles. The retention time is a function of tank depth.
units can be rectangular in design but are usually circular, resembling a
primary clarifier or thickener. They are often single-stage units.
air flotation (IAF) is another method of decreasing particle density by
attaching air bubbles to the particles; however, the method of generating the
air bubbles differs. A mechanical action is employed to create the air bubbles
and their contact with the waste contaminants. The most common methods use
high-speed agitators or recycle a slip stream through venturi nozzles to entrain
air into the wastewater.
contrast to DAF units, IAF units are usually rectangular and incorporate four or
more air flotation stages in series. The retention time per stage is
significantly less than in DAF circular tanks.
in gravity settling, the diameter of the particle plays an important role in
separation. Polyelectrolytes may be used to increase effective particle
diameters. Polymers are also used to destabilize oil-water emulsions, thereby
allowing the free oil to be separated from the water. Polymers do this by charge
neutralization, which destabilizes an oil globule surface and allows it to
contact other oil globules and air bubbles. Emulsion breakers, surfactants, or
surface-active agents are also used in air flotation to destabilize emulsions
and increase the effectiveness of the air bubbles.
is employed in waste treatment wherever suspended solids must be removed. In
practice, filtration is most often used to polish wastewater following
treatment. In primary waste treatment, filters are often employed to remove oil
and suspended solids prior to biological treatment. More commonly, filters are
used following biological treatment, prior to final discharge or reuse.
used for waste treatment may be designed with single-, dual-, or multimedia and
may be of the pressure or gravity type.
REMOVAL OF SOLUBLE CONTAMINANTS
pH Adjustment-Chemical Precipitation
industrial wastewaters contain high concentrations of metals, many of which are
soluble at a low pH.
of pH precipitates these metals as metal oxides or metal hydroxides. The pH must
be carefully controlled to minimize the solubility of the contaminant. As shown
in Figure 37-4, some compounds, such as zinc, are amphoteric and redissolve at a
high pH. Chemicals used for pH adjustment include lime, sodium hydroxide, and
precipitation of soluble ions often occurs as the result of pH adjustment.
Contaminants are removed either by chemical reaction leading to precipitation or
by adsorption of ions on an already formed precipitate.
Biological Oxidation-Biochemical Reactions
of the most common ways to convert soluble organic matter to insoluble matter is
through biological oxidation. Soluble organics metabolized by bacteria are
converted to carbon dioxide and bacterial floc, which can be settled from
microorganisms feed on dissolved and suspended organic compounds. This natural
biodegradation can occur in streams and lakes. If the assimilative capacity of
the stream is surpassed, the reduced oxygen content can cause asphyxiation of
fish and other higher life forms. This natural ability of microorganisms to
break down complex organics can be harnessed to remove materials within the
confines of the waste plant, making wastewater safe for discharge.
biodegradable contaminants in water are usually measured in terms of biochemical
oxygen demand (BOD). BOD is actually a measure of the oxygen consumed by
microorganisms as they assimilate organics.
metabolize oxygen along with certain nutrients and trace metals to form cellular
matter, energy, carbon dioxide, water, and more bacteria. This process may be
represented in the form of a chemical reaction:
Food (organic compounds)
+ Carbon dioxide
of the water depends on minimizing the amount of "food"
(organic compounds) that remains after treatment. Therefore, biological waste
treatment facilities are operated to provide an environment that will maximize
the health and metabolism of microorganisms. An integral part of the biological
process is the conversion of soluble organic material into insoluble materials
for subsequent removal (Figure 37-5). An overview of factors involved in
biological oxidation is given in Table 37-1.
Open Lagoon Biological Oxidation
organic loads are low and sufficient land area is available, open lagoons may be
used for biological treatment. Lagoons provide an ideal habitat for
microorganisms. Natural infiltration of oxygen is sufficient for biological
oxidation if the organic loading is not too high. However, mechanical aeration
(Figure 37-6) is often used to increase the ability to handle a higher loading.
Lagoons are nothing more than long-term retention basins.
Ordinarily shallow in depth, they depend on surface area, wind, and wave action
for oxygen transfer from the atmosphere. Depending on the influent BOD loading
and oxygen transfer, lagoons may be aerobic or anaerobic. Lagoons are used
primarily for low BOD wastes or as polishing units after other biological
Aerated Lagoons. As BOD loading increases, naturally occurring surface oxygen transfer becomes insufficient to sustain aerobic bacteria. It then becomes necessary to control the environment artificially by supplying supplemental oxygen. Oxygen, as air, is introduced either by mechanical agitators or by blowers and subsurface aerators. Because energy must be expended, the efficiency of the oxygen transfer is a consideration. Therefore, although unaerated lagoons are typically 3-5 ft deep, allowing large surface areas for natural transfer, aerated lagoons are usually 10-15 ft deep in order to provide a longer, more difficult path for oxygen to escape unconsumed. Aerated lagoons also operate with higher dissolved oxygen content.
Facultative Lagoons. Lagoons without mechanical aeration are usually populated by facultative organisms. These organisms have the ability to survive with or without oxygen. A lagoon designed specifically to be facultative is slightly deeper than an unaerated lagoon. Influent suspended solids and solids created by the metabolism of the aerobic bacteria settle to the bottom of the lagoon where they undergo further decomposition in an anaerobic environment.
Activated Sludge Oxidation
to the reaction presented previously, control of contaminant oxidation at high
BOD loadings requires a bacteria population that is equal to the level of food.
This need is the basis for the activated sludge process.
the activated sludge process, reactants, food, and microorganisms are mixed in a
controlled environment to optimize BOD removal. The process incorporates the
return of concentrated microorganisms to the influent waste.
are separated from wastewater leaving an aeration basin and
reintroduced to the influent, they continue to thrive. The recirculated bacteria
continue to oxidize wastewater contaminants, and if present in sufficient
quantity, produce a relatively low BOD effluent water.
the activated sludge process incorporates the return of concentrated
microorganisms, it must include a process for microorganism concentration and
removal. This process includes an aeration stage and a sedimentation stage
(Figure 37-7). Because suspended solids are considered wastewater contaminants,
the sedimentation stage accomplishes two functions: concentration of bacteria
and removal of solids.
operating parameters that affect the performance of any activated sludge process
are BOD, microorganisms, dissolved oxygen, retention time, nutrient
concentration, and the external influences of temperature and pH. In order to
understand the various activated sludge designs, it is necessary to examine the
relationship between available food and bacteria population.
a seed culture of bacteria is introduced into a fixed amount of food, the
conditions shown in Figure 37-8 are created.
Initially, excess food is present;
therefore, the bacteria reproduce in a geometric fashion. This is termed the
"log growth phase." As the population increases and food decreases, a
plateau is reached in population. From the inflection point on the curve to the
plateau, population is increasing but at a decreasing rate. This is called the
"declining growth phase." Once the plateau is crossed, the bacteria
are actively competing for the remaining food. The bacteria begin to metabolize
stored materials, and the population decreases. This area of the curve is termed
"endogenous respiration." Eventually, the bacteria population and BOD
are at a minimum.
sludge is a continuous, steady-state process, each plant operates at
some specific point on this curve, as determined by the oxidation time provided.
The point of operation determines the remaining bacteria population and BOD
of an activated sludge plant requires the integration of mechanical,
operational, and chemical approaches for the most practical overall program.
Mechanical problems can include excessive hydraulic loading, insufficient
aeration, and short-circuiting. Operational problems may include spills and
shock loads, pH shocks, failure to maintain correct mixed liquor concentration,
and excessive sludge retention in the clarifier.
chemical treatment programs are described below. Table 37-2 presents a
comparison of various treatment schemes.
Sedimentation. Because activated sludge depends on microorganism recirculation, sedimentation is a key stage. The settleability of the biomass is a crucial factor. As bacteria multiply and generate colonies, they excrete natural biopolymers. These polymers and the slime layer that encapsulates the bacteria influence the flocculation and settling characteristics of bacteria colonies. It has been determined empirically that the natural settleability of bacteria colonies is also a function of their position on the time chart represented in
Figure 37-8. Newly formed colonies in the log growth phase are relatively non-settleable. At the end of the declining growth phase and the first part of the endogenous phase, natural flocculation is at an optimum. As the endogenous phase continues, colonies break up and floc particles are dispersed, decreasing the biomass settleability.
microbes are eventually able to break down most complex organics and can
tolerate very poor environments, they are very intolerant of sudden changes in
pH, dissolved oxygen, and the organic compounds that normally upset an activated
sludge system. These upsets normally result in poor BOD removal and excessive
carryover of suspended solids (unsettled microorganisms) in the final effluent.
Aeration. Aeration is a critical stage in the
activated sludge process. Several methods of aeration are used:
High Rate Aeration. High rate aeration operates in the log growth phase. Excess food is
provided, by recirculation, to the biomass population. Therefore, the effluent from this design contains appreciable levels of BOD (i.e., the oxidation process is not carried to completion). Further, the settling characteristics of the biomass produced are poor. High sludge return rates are necessary to offset poor settling and to maintain the relatively high biomass population. Poor settling increases the suspended solids content of the effluent. The relatively poor effluent produced limits this design to facilities which need only pretreatment before discharge to a municipal system. The advantage of high rate aeration is low capital investment (i.e., smaller tanks and basins due to the short oxidation time).
Conventional Aeration. The most common activated sludge design used by municipalities and industry operates in the endogenous phase, in order to produce an acceptable effluent in BOD and TSS levels. Conventional aeration represents a "middle of the road" approach because its capital and operating costs are higher than those of the high rate process, but lower than those of the extended aeration plants. As shown in
Figure 37-8, the conventional plant operates in the area of the BOD curve where further oxidation time produces little reduction in BOD. Natural flocculation is optimum, so the required sedimentation time for removal of suspended solids from the effluent is minimized.
Extended Aeration. Extended aeration plants operate in the endogenous phase, but use longer periods of oxidation to reduce effluent BOD levels. This necessitates higher capital and operating costs (i.e., larger basins and more air). In conjunction with lower BOD, extended aeration produces a relatively high suspended solids effluent when optimum natural settling ranges are exceeded.
aeration designs may be necessary to meet effluent BOD requirements when the
influent is relatively concentrated in BOD or the wastes are difficult to
biodegrade. Because extended aeration operates on the declining side of the
biomass population curve, net production of excess solids is minimized, as shown
in Table 37-3. Therefore, savings in sludge handling and disposal costs may
offset the higher plant capital and operating costs required for extended
Step Aeration/Tapered Aeration. In a plug flow basin, the head of the basin receives the waste in its most concentrated form. Therefore, metabolism and oxygen demand are greatest at that point. As the waste proceeds through the basin, the rate of oxygen uptake (respiration rate) decreases, reflecting the advanced stage of oxidation.
aeration and step aeration reduce this inherent disadvantage. Tapered aeration
provides more oxygen at the head of the basin and slowly reduces oxygen supply
to match demand as the waste flows through the basin. This results in better
control of the oxidation process and reduced air costs.
Step aeration modifies the introduction of influent
waste. The basin is divided into several stages, and raw influent is introduced
to each stage proportionately. All return microorganisms (sludge) are introduced
at the head of the basin. This design reduces aeration time to 3-5 hr, while BOD
removal efficiency is maintained. The shorter aeration time reduces capital
expenses because a smaller basin can be used. Operating costs are similar to
those of a conventional plant.
Contact Stabilization. Due to the highly efficient sorptive capabilities of activated biomass, the time necessary for the biomass to "capture" the colloidal and soluble BOD is approximately 30 min to 1 hr. Oxidation of fresh food requires the normal aeration time of 4-8 hr. In the contact stabilization design, relatively quick sorption time reduces aeration tank volume requirements. The influent waste is mixed with return biomass in the initial aeration tank (or contact tank) for 30-90 min. The entire flow goes to sedimentation, where the biomass and its captured organics are separated and returned to a reaeration tank. In the reaeration tank the wastes undergo metabolism at a high biomass population. The system is designed to reduce tank volume by containing the large majority of flow for a short period of time.
process is not generally as efficient in BOD removal as the conventional plant
process, due to mixing limitations in the contact basin. Operating costs are
equivalent. Due to the unstabilized state of the biomass at sedimentation,
flocculation is inferior. Suspended solids in the effluent are problematic.
this design exposes only a portion of the active biomass to the raw effluent at
a time, it is less susceptible to feed variations and toxicants. For this reason
it can be beneficial for treatment of industrial wastes.
Pure Oxygen Sludge Processes. Oxygen supply and transfer often become limiting factors in industrial waste treatment. As the name implies, pure oxygen activated sludge processes supply oxygen (90-99% O2) to the biomass instead of air. The increased partial pressure increases transfer rates and provides several advantages. Comparable or higher BOD removal efficiencies are maintained at higher BOD influent loadings and shorter retention times. Generally, aeration time is 2-3 hr. A further advantage is the production of lower net solids per pound of BOD removed. Therefore, sludge disposal costs are reduced.
units are usually enclosed. Normally, three or four concrete box stages in
series are provided for aeration. The raw wastewater, return biomass, and pure
oxygen enter the first stage. Wastewater passes from stage to stage in the
atmosphere flows over the open surface of each stage to the last stage, from
which it is vented to control the oxygen content. Oxygen purity and the demand
for oxygen decline through the stages. Each stage contains a mechanical agitator
for mixing and oxygen transfer. By design, each stage is completely mixed. After
aeration, the waste flows to a conventional sedimentation stage. BOD and TSS
removals are usually somewhat better than in a conventional aeration system.
Chemical Treatment Programs. The following additives represent a variety of chemical programs that may be used to address problems and improve system efficiency.
Essential Nutrients. Nutrients, particularly nitrogen and phosphorus, may be added to ensure complete digestion of organic contaminants.
Polymers. Polymer feeding improves the settling of suspended solids. Cationic polymers can increase the settling rate of bacterial floc and improve capture of dispersed floc and cell fragments. This more rapid concentration of solids minimizes the volume of recycle flow so that the oxygen content of the sludge is not depleted. Further, the wasted sludge is usually more concentrated and requires less treatment for eventual dewatering. Polymers may also be used on a temporary basis to improve the removal of undesirable organisms, such as filamentous bacteria or yeast infestations, that cause sludge bulking or carryover of floating clumps of sludge.
Oxidizing Agents. Peroxide, chlorine, or other agents may
be used for the selective oxidation of troublesome filamentous bacteria.Antifoam
Agents. Antifoam agents may be used to control excessive foam.
In addition to antifoam agents, coagulants may be fed continuously to improve
efficiency, or to address particularly difficult conditions. They may also be
used intermittently to compensate for hydraulic peak loads or upset conditions.
Fixed Media Biological Oxidation
contrast to activated sludge, in which the bio-mass is in a fluid state, fixed
media oxidation passes influent wastewater across a substructure laden with
fixed biomass. The parameters for healthy microorganisms remain the same, except
the manner in which food and microorganisms come into contact.
media designs allow a biological slime layer to grow on a substructure
continually exposed to raw wastewater. As the slime layer grows in thickness,
oxygen transfer to the innermost layers is impeded. Therefore, mixed media
designs develop aerobic, facultative, and anaerobic bacteria as a function of
the thickness of the slime layer. Eventually, either because of size and
wastewater shear or the death of the microorganisms, some of the slime layer
sloughs off. In a continuous process, this constantly sloughing material is
carried to a sedimentation stage, where it is removed. There are no provisions
to recycle the microorganisms, because return sludge would plug the fixed media
structure. In fact, media plugging and lack of oxygen transfer are the primary
difficulties encountered with fixed media designs. Plugging problems can be
alleviated by increased wastewater shear. This is normally accomplished by
recycling of a portion of the effluent wastewater.
Trickling Filters. Trickling filters are not really filters but a filter-like form of fixed media oxidation. Wastewater is sprayed over a bed of stones, 3-5 in. in diameter. Bed depths range from 5 to 7 ft. Because air contact is the sole means of oxygen transfer, microorganisms become more oxygen deficient as depth increases.
filters can be classified by hydraulic loading as low-rate, high-rate, or
roughing. Due to inherent oxygen transfer difficulties, even low rate filters
cannot achieve the BOD removal possible in conventional activated sludge
systems. Industrial trickling filters are usually followed by an activated
sludge unit. They may be used as a pretreatment step before discharge to a
municipal sewage system.
Biological Towers. Another form of fixed media filter uses synthetic materials in grid fashion as a substructure for biological growth. The high porosity available with artificially designed media alleviates the oxygen transfer problems of trickling filters and allows greater bed depths. Bed depths of up to 20 ft with adequate oxygen allow longer contact and consequently better BOD removal.
Biodiscs. Biodiscs are a recently developed form of fixed media oxidation. The media is fixed to a rotating shaft that exposes the media alternately to food (wastewater) and oxygen (atmosphere). Design parameters include speed of rotation, depth of the wastewater pool, porosity of the synthetic media, and number of series and parallel stages. These units circumvent the oxygen limitations of the trickling filter and therefore provide BOD removal comparable to conventional activated sludge systems. Solids produced are easily settled in the sedimentation stage, provi-ding acceptable TSS levels in the effluent. Little operational attention is required.
SOLID WASTE HANDLING
treatment is a concentration process in which waterborne contaminants are
removed from the larger wastewater stream and concentrated in a smaller side
stream. 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.
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.
Anaerobic Digestion. Anaerobic digestion takes place in an enclosed tank, as
depicted in Figure 37-9. The biochemical reactions take place in the following phases:
solids are decreased due to the conversion of biomass to methane and carbon
dioxide. The methane can be recovered for its heating value.
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.
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.
Composting. A natural digestion process, composting usually incorporates sludge material that later will be applied to farmland. Sludge is combined with a bulking material, such as other solid wastes or wood chips, and piled in windrows. Aeration is provided by periodic turning of the sludge mass or by mechanical aerators. The energy produced by the decomposition reaction can bring the waste temperature to 140-160°F, destroying pathogenic bacteria. At the end of the composting period, the bulking material is separated, and the stabilized sludge is applied to land or sent to a landfill.
sludge from a final liquid-solids separation unit may contain from 1 to 5% total
suspended solids. Figure 37-10 shows the relationship between the volume of
sludge to be handled and the solids content in the sludge. 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 colloidal (less than 1
µm), intermediate, and large particles (greater than 200 µm).
The large particles, or flocs, are usually compressible. Under an energy
gradient, these large flocs compress and prevent water from escaping.
The small particles
also participate in this mechanism, plugging the pores of the sludge
cake, as shown in Figure 37-11. The pressure drop through the sludge
cake, due to the decrease in porosity and pore sizing, exceeds available
energy, and dewatering ceases.
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 FeCl3, 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 FeCl3,
lime, and either cationic or anionic polymers.
Belt Filter Press. Belt filter presses have been used in Europe since the 1960's and in the United States since the early 1970's. They were initially designed to dewater paper pulp and were subsequently modified to dewater sewage sludge.
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.
Figure 37-12 shows these zones in a simplified schematic of a belt filter press.
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.
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 5-10%. 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.
Compression (Low-Pressure) Area. The compression, or low-pressure, area is the point at which the sludge is "sandwiched" between the upper and lower belts. A firm sludge cake is formed in this zone in preparation for the shear forces encountered in the high-pressure zone.
Shear (High-Pressure) Zone. In the shear, or high-pressure, zone, forces are exerted on the sludge by the movement of the upper and lower belts, relative to each other, as they go over and under a series of rollers with decreasing diameters. Some manufacturers have an independent high-pressure zone which uses belts or hydraulic cylinders to increase the pressure on the sludge, producing a drier cake. A dry cake is particularly important for plants that use incineration as the final disposal.
belts are usually woven from monofilament polyester fibers. Various weave
combinations, air permeabilities, and particle retention capabilities are
available. These parameters greatly influence press performance.
cationic polymers are used for sludge conditioning. A two-polymer system is
often used on a belt filter press to improve cake release from the upper
dewatering belt. The polymer must be selected carefully to ensure optimum
by proper ventilation, by ensuring that the sludge does not turn
septic, and by the use of added chemicals, such as potassium permanganate or
ferric sulfate, to neutralize the odor-causing chemicals.
Screw Press. A recent development in sludge dewatering equipment, used primarily in the pulp and paper industry, is the screw press. Screw presses are most effective for primary sludges, producing cake solids of 50-55%, but are also appropriate for primary and secondary blended
is conditioned and thickened prior to dewatering. The conditioned sludge enters
one end of the machine, as shown in Figure 37-13. A slowly rotating screw,
analogous to a solid bowl centrifuge, conveys and compresses the solids.
screw has the same outer diameter and pitch for the entire length of the press.
In some models, the diameter of the screw shaft increases toward the discharge
end of the screw press to enhance dewatering. The compression ratio (the ratio
of free space at the inlet to the space at the discharge end of the screw) is
selected according to the nature of the material to be dewatered and the
dewatering requirement. Dewatered cake is discharged as it is pressed against
the spring or hydraulically loaded cone mounted at the end of the screw press.
drum of the screw press consists of a fine strainer screen, a thicker punched
holding plate, and a reinforcement rib.
Filtrate is collected in the collecting pan located under
the screw press, and the cake is transported to the next stage.
Vacuum Filters. Vacuum filtration uses various porous materials as filter media, including cloth, steel mesh, and tightly wound coil springs. Under an applied vacuum, the porous medium retains the solids, but allows water to pass through. The relative importance of cake dryness, filtrate quality, and filter cake yield can vary from one system to another.
drum speed allows more time for drying of the sludge to increase
cake dryness. However, this also decreases the filter cake yield, defined as
pounds of dry solids per hour per square foot of filter area. Polymers can help
produce a drier cake without the problem of a lower filter cake yield. Synthetic
polymers improve cake dryness by agglomerating sludge particles that may hinder
the removal of water. This agglomeration also increases the solids capture
across the unit, which results in a higher-quality filtrate.
Centrifuges. Centrifugal force, 3500-6000 times the force of gravity, is used to increase the sedimentation rate of solid sludge particles.
most common centrifuge found in waste treatment dewatering applications is the
continuous bowl centrifuge (Figure 37-14). 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.
operation of centrifugal dewatering equipment is a compromise between centrate
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 centrate. The addition of polymers, with their
ability to agglomerate fine particles, can result in increased production rates
without a loss in centrate quality.
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.
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 pressures of up
to 25 psig 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 from to 2 hr.
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 5-10 psig, polymers can produce a
rigid floc and eliminate fine particles. 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. 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.
is drained from the sludge cake by gravity through the sand and gravel bed. This
process is complete within the first 2 days. All additional drying occurs by
evaporation, which takes from 2 to 6 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
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
disposal methods include reclamation, incin-eration, land application, and
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 CaO, return of steel mill thickener
sludge to the sinter plant, and marketing of by-product metallic salts for
wastewater treatment use.
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.
combustion methods are available, including hogged fuel boilers, wet air
oxidation and kiln, multiple hearth furnace, and fluidized bed combustion
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.
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.
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, defined as having a permeability of 10-7 cm/sec, 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 treat-ment systems are typically required for new and remediated
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
Many governmental regulations have been established in recent
years for the protection of the environment. The Clean Water Act and
the Resource Conservation and Recovery Act are among the most significant.
Clean Water Act
Clean Water Act (CWA) of 1972 established regulations for wastewater discharge,
provided funding for Publicly Owned Treatment Works (municipal waste treatment
plants), and authorized the National Pollutant Discharge Elimination Systems (NPDES)
to regulate and establish wastewater discharge permits for industrial and
Resource Conservation and Recovery Act
Resource Conservation and Recovery Act (RCRA) of 1976 provided regulations for
management of hazardous solid wastes, cleanup of hazardous waste sites, waste
minimization, underground storage, and groundwater monitoring.