Secondary Wastewater Treatment (part 2)

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Activated Sludge Process

Control Concepts

Common control methods used for the activated sludge process include:

• Solids retention time (SRT)

• Food-to-microorganism ratio (F/M)

• Constant mixed liquor suspended solids (MLSS)

• Return activated sludge control

Objectives of activated sludge secondary treatment are to biologically oxidize waste organic content aerobically and to remove excess new cell growth solids before discharge to receiving waters. Each treatment plant operates within a range of process control parameters based on original plant design that tends to keep the biological process in a steady state condition, as long as raw waste influent characteristics do not change too abruptly. By maintaining process parameters within a narrow range, swings in process efficiency that may result from shock loads or upsets are minimized, and continuously high organic removal efficiency is assured.

Solids Retention Time

Solids retention time (SRT), also called mean cell residence time (MCRT) or sludge age, is calculated by dividing the total quantity of sludge in the aeration basin and clarifier by daily sludge losses through waste activated sludge and effluent. Equation shows the calculation:

SRT = XV XV XQ XQ xx ee 11 22

where SRT = solids retention time, days

X1 = MLSS in aeration basin, mg/L

X2 = MLSS in clarifier, mg/L

Xx = MLSS in waste activated sludge, mg/L

Xe = MLSS in effluent, mg/L

V1 = volume of aeration basin, gal (m3 )

V2 = volume of clarifier, gal (m3 )

Qx = effluent flow rate, gpd (m3 /d)

Qe = waste activated sludge flow rate, gpd (m3 /d)

The activated sludge system can be controlled based on keeping the SRT value at a constant level. (See Tbl. for recommended ranges for activated sludge processes.) If SRT overly exceeds the recommended range, which directly means scarce food condition or low food-to-microorganism (F/M) ratio, sludge floc can be dismantled, and pin floc can be formed. This condition is often accompanied by solids loss and an increase in effluent turbidity from the secondary clarifier.

TBL. 9 Summary of Operating Data for Activated Sludge Processes

When the system is operated at lower than recommended SRT, a condition commonly called "young sludge" may result. This light, fluffy, buoyant sludge, also called straggler floc, settles slowly. This is witnessed in a clarifier when these fluffy floc particles are pulled over weirs even though the effluent may be relatively clear. This condition may appear in the aeration basin as white billowy foam. These conditions are typical of activated sludge systems just after startup. In addition, SRT lower than 5 days is not sufficient to grow enough slowly growing autotrophic microorganisms that oxidize nitrogen.

Consequently, nitrification is not possible at low SRT.

The main variable in SRT control is sludge wasting rate. Generally, operation involves finding an SRT level at which the plant achieves optimum effluent quality. When the SRT value has been reduced to proper levels and conditions in the treatment system improved, the operator stops or decreases the sludge-wasting rate to maintain constant SRT. There are conditions and periods depending on loading rates, changes in flow rates, and presence of toxic shocks, where SRT levels should be increased or decreased.

Food-to-Microorganism Ratio

A term for expressing organic loading of an activated sludge process is food-to-microorganism ratio (F/M), and is a critical factor in process design and operation, especially in determining aeration basin volume.

F/M is usually defined as:

FM / = () - QS S / XV AA

where F/M = food-to-microorganism ratio, lb BOD/[lb MLVSS · d] × [kg BOD/(kg MLVSS · d)] Q0 = influent flow rate, gpd (m3 /d)

S0 = influent BOD5 , mg/L

Se = effluent BOD5 , mg/L

XA = mixed liquor volatile suspended solids (MLVSS) in aeration basin, mg/L

V A = aeration basin volume, gal (m3 ) If only COD is available for domestic wastewater, divide the COD by 1.5 to obtain approximate BOD5

. Similarly, MLSS can be multiplied by 0.75 to obtain an approximate MLVSS. BOD/COD and MLSS/MLVSS ratios for industrial wastewater can vary significantly depending on the type of industrial process.

F/M and MLVSS are important design criteria for sizing the aeration basin and along with sludge age are important operating criteria.


Suspended solids level is one of the most important control parameters in biological wastewater treatment processes. It is not only directly related with sludge settling properties and effluent quality, but also related with F/M ratio that is in turn related with all aspects of sludge properties.

In general, MLSS represent total suspended solids, irrespective of biological activity. Measurement is expressed as an empirical test result that reports the quantity of suspended solids carried in the aeration basin. Usually, glass filters having a pore size of 1.2 µm are used.

On the other hand, MLVSS represent the organic portion of MLSS, which is used to represent biomass. This is also expressed as an empirical test result (the amount of solids that was lost in volatilization at 550°C); this test reports roughly the portion of MLSS that has the active microbial population used to absorb and degrade soluble organic matter in wastes.

The activated sludge system can be controlled based on a constant MLSS level much the same way as the SRT control approach.

(See Tbl. for recommended ranges for activated sludge processes). Similar conditions occur in the process due to MLSS concentrations higher and lower carried in the activated sludge system (as described for respective conditions for SRT levels). As with SRT control, the main variable for maintaining constant MLSS is the sludge-wasting rate. After an acceptable MLSS level is determined for operation of the system, the sludge-wasting rate is controlled to maintain this level. When the MLSS level increases above the determined optimum range, the plant operator starts or increases the sludge-wasting rate. When the MLSS level decreases below the range, the operator stops or decreases the sludge-wasting rate, to maintain a constant MLSS level in the system and F/M in the optimum range.

Sludge Blanket Depth

Sludge blanket depth in the clarifier should be measured at the same time each day and at the same point on the clarifier bridge.

The best time is during the period of maximum daily flow, because the clarifier is operating under the highest solids loading rate.

Adjustments in the recycle activated sludge (RAS) flow rate should be needed only occasionally if the activated sludge process is operating properly.

An additional advantage of monitoring sludge blanket depth is that problems, such as improperly operating sludge collection equipment, can be observed due to irregularities in blanket depth. A plugged pick-up on a clarifier sludge collection system would cause sludge depth to increase in the area of the pick-up, and decrease in areas where properly operating pick-ups are located. These irregularities in sludge blanket depth are easily monitored by measuring profiles of blanket depth across the clarifier.

Other Parameters

Used to Evaluate Performance Other parameters can be used to track performance of the activated sludge process. These parameters are also effective in assessing health of microorganisms and can be used to determine where problems exist. Some common parameters are:

• Sludge volume index (SVI)

• Oxygen uptake rate

• Microscopic examination

• Aerator loading

• Detention time

• Mixed liquor dissolved oxygen level Sludge Volume Index This index is defined as the volume (in milliliters) occupied by 1 g of activated sludge mixed liquor solids, dry weight, after settling for 30 min in a 1000 mL graduated cylinder. In practice, it is taken to be the percent volume occupied by the sludge in a mixed liquor sample (taken at the outlet of the aeration tank) after 30 min of settling, divided by the suspended solids concentration of the mixed liquor. The formula is expressed as:

SVI MLSS = Vs/ (11) where SVI = sludge volume index, mL/g MLSS

Vs = volume of settled sludge after 30 min, mL/L

MLSS = mixed liquor suspended solids, g/L SVI has been used as an indication of the settling characteristics of the sludge. However, SVI that is characteristic of good settling sludge varies with the type of industrial waste and concentration of mixed liquor solids, and observed values at a given plant should not be compared with those reported for other plants or in the literature.

Typical SVI for good settling sludge with mixed liquor concentrations in the range of 1500 to 3500 mg/L are:

• SVI of 80 to 120 is normal and considered good settling.

• SVI greater than 120 is an indication of possible bulking in the clarifier.

• SVI less than 80 is an indication of very compact and heavy floc.

Dissolved Oxygen Uptake Dissolved oxygen (DO) uptake is determined from a short duration DO test measurement in a standard 300 mL BOD5 bottle, using oxygen saturated samples of fresh aerator mixed liquor, return sludge, bioreactor influent, or a mixture of bioreactor influent, plus a portion of return sludge. Results are typically taken once or twice a shift by treatment plant operators, and the results are usually graphed.

OUR DO DO h =- ()(min/)/ 010 60 10 (12)

MLRR OUR MLVSS = / (13) where OUR = oxygen uptake rate, mg O2 /[L · h] DO0 = initial DO at time zero, mg/L

DO10 = DO after 10 min, mg/L

MLRR = mixed liquor respiration rate, mg O2 /[g MLVSS· h]

MLVSS = mixed liquor volatile suspended solids, g/L

Results are typically used to evaluate factors such as aeration system capacity limitations, mixed liquor solids levels, bioreactor raw water treatability, immediate dissolved oxygen demand, reactivity of raw wastes, and return sludge thickening time limits for maintaining viable mixed liquor microorganisms in the secondary clarifier.

The respiration rate tells an operator about the state of health of activated sludge mixed liquor. The respiration rate of a normal steady-state operational activated sludge plant changes from season to season.

Interpretation of these results is straightforward. A typical respiration rate in a sludge sample from the beginning of the aeration tank is between 12 and 20 mg O2/[g MLVSS · h]. Higher rates indicate high organic loading, while lower rates indicate low loading.

Respiration rates higher than normally observed suggest increases in reactor BOD loadings or F/M. High respiration rates [usually more than 35 to 45 mg O2/(g MLVSS · h)] are usually characteristic of young mixed liquor that is under oxidized. Comparison of oxy gen uptake rate of mixed liquor with that of secondary effluent usually shows high effluent uptake rate [20 to 30 mg O2/(L · h)] for secondary effluent is high). High results should be compared with settling or compaction rates of mixed liquor solids and foaming tendencies, since these results tend to be related when mixed liquor solids are young and under aerated.

Lower than normal respiration rates indicate one of three things:

1. SRT is longer than usual.

2. BOD loading has dropped off, decreasing F/M.

3. Some type of biological inhibitory agent is present in the waste.

Low respiration rate should be checked right away with oxy gen uptake results for all four samples. Healthy activated sludge has an oxygen uptake rate of 8 to 20 mg O2 /(L · h) for a sample of mixed liquor. When levels drop below this range, it can be an indication of biological toxicity in the reactor, dictating the need for specific action.

Mixed Liquor Dissolved Oxygen Mixed liquor DO level in activated sludge aeration basins is controlled by:

• Organic loading directly influences oxygen uptake rate, which indirectly affects DO level.

• Aeration source and level directly influence DO level.

• Mixed liquor solids inventory indirectly influences oxygen uptake rate and F/M ratio, which in turn indirectly affect mixed liquor DO level.

Dissolved oxygen levels in the effluent from the aeration tank are generally maintained between 1.0 and 2.0 mg/L or higher, depending on the process.

Nutrient Removal in Activated Sludge

In water environments such as rivers, lakes, and oceans, the most stringent nutrient for algal growth is phosphorous. Therefore, controlling phosphorous in the water environment is the single most effective means to maintain clean water and prevent eutrophication (algae growth). In addition to phosphorous, nitrogen is the next most stringent nutrient, though some algae can fix gaseous nitrogen in air.

While there are many forms of nitrogen in terms of oxidation state, ammonia nitrogen is the most readily utilized form for most organisms that contaminate water. In addition, ammonia nitrogen consumes dissolved oxygen and may suffocate fish and higher life forms, when it is biologically oxidized. Both are important, although controlling ammonia nitrogen is given higher priority than controlling nitrate nitrogen.

In biological wastewater treatment, some part of phosphorous and nitrogen can be converted to biomass and eventually removed through sludge wastage. Microorganisms treating wastewater contain 1.7% phosphorous and 8.7% nitrogen. If SRT and other conditions are properly managed, the majority of ammonia and organic nitrogen can be oxidized to nitrate, which is less harmful to the water environment.

Phosphorous and nitrogen removal can be further enhanced biologically using cyclic dissolved oxygen conditions.

Phosphorous Removal

If sludge containing many kinds of different microorganisms is circulated periodically between low anaerobic and high DO conditions, some species that can adapt well in this cyclic DO condition will have advantage over other microorganisms. While most heterotrophic microorganisms can make energy under the oxygen rich environment, some microorganisms [polyphosphate accumulating organisms (PAO)] can make energy without oxygen by hydrolyzing the polyphosphate accumulated in the cell.

If PAO is circulated between DO rich and anaerobic conditions periodically, they accumulate excess phosphorous during DO rich conditions to prepare for the DO scarce conditions. During DO scarce conditions, PAO absorb fatty acids in the liquid phase and store them in the form of polyhydroxybutyrate (PHB), which is used to produce new PAO during DO rich conditions.

Phosphorous accumulation during DO rich conditions is often called "luxury uptake." Under aerobic conditions, more phosphorous is accumulated by PAO to prepare for anaerobic conditions. Moreover, during anaerobic conditions, PAO have a clear advantage in survival over other microorganisms.

Phosphorous can be also removed chemically. Addition of chemicals to the secondary clarifier for phosphate precipitation is commonly practiced. A mineral salt addition involving Fe+3 or Al +3 combines with soluble orthophosphate available and precipitates as insoluble metal phosphates. Section 24 describes dosages for phosphorus removal by chemical addition. However, adding acid salts of these metals neutralizes alkalinity of waste and drops pH.

Reduction in alkalinity sometimes inhibits biological treatment in the activated sludge process. In addition, formation of a substantial amount of insoluble iron or aluminum hydroxide can increase sludge handling and dewatering costs significantly.

In these cases, where chemical phosphorus removal is intended to take place in activated sludge, tests should be run to see if sodium aluminate can be substituted for acid metal salts of iron or aluminum. Reactions and products of typical phosphorus removal agents for this type of application are summarized in Section 24. Solubility of metal salts used in phosphorus precipitation is strongly pH dependent. Ill. 24.1 shows solubility of iron and aluminum phosphate versus pH.

Nitrogen Removal

Nitrogen removal can be achieved as follows:

NH NO NO N gas OO BOD 4232 22 +-- ?? ??? ??? ?? ? ( ) (14) Nitrification occurs by autotrophs in aerobic conditions with consumption of dissolved oxygen. All biological processes having oxygen rich stages and long sludge age can oxidize ammonia nitrogen to nitrate as follows:

23 224 42 22 NH O NO H O H Nitrosomonas +- + +? ? ???? + + (15)

22 22 3 NO O NO Nitrobacter -- +? ? ???? (16) Denitrification occurs in the absence of free oxygen (anoxic condition). Most heterotrophs can respire with consumption of combined oxygen in the form of nitrate, nitrite, hypochlorite, and the like.

22 25 3222 NO H N gas H O O -+ +?? ??++ ( ) . (17) In the first nitrification process [Eqs. (15) and (16)], autotrophic microorganisms (nitrifiers) oxidize ammonia nitrogen to nitrate with consumption of DO. Then [Eq. (17)] nitrate is reduced to nitrogen gas in anoxic tanks, where heterotrophic microorganisms deliver oxygen from combined oxygen sources such as nitrate due to a lack of elemental oxygen.

In terms of kinetics, nitrification is much slower than denitrification.

Autotrophs not only grow slower than heterotrophs, but the amount of food for autotrophs (nitrogen) is scarcer than that for heterotrophs (BOD). Autotroph concentration is normally less than 10% of the total bacterial population. Consequently, if SRT is lower than 5 days, autotrophs rarely build up to a sufficient level due to washout.

Moreover, autotrophs are more sensitive to temperature. Nitrification can be severely affected below 50ºF (10ºC), except in cases where MLSS is very high such as in membrane bioreactor (MBR) processes in which MLSS is around 10 g/L.

Since combined oxygen in nitrate is utilized during BOD oxidation by heterotrophs, a certain amount of BOD is necessary to reduce nitrate. Theoretically, the BOD/TKN (total Kjeldahl nitrogen) ratio should be at least 3.4:1 to denitrify all nitrogen. If the ratio is lower than 3.4:1, nitrogen removal is limited to some extent depending on BOD deficiency. In these cases, a source of carbon such as methanol is added to the denitrification basin.

In the activated sludge process, pH in the aeration basin often decreases below the optimum range (6.5 to 7.5) mainly due to formation of nitrates during oxidation of TKN (ammonia nitrogen plus organic nitrogen) contained in the influent. In theory, 1 mg nitrogen can consume 3.6 mg alkalinity as CaCO3 , if it is not denitrified.

However, when denitrification is performed, net acid formation by nitrification can be reduced, and pH can be maintained more easily.

Aerated Lagoons

Aerated lagoons or ponds evolved from facultative stabilization ponds when surface aerators were installed to overcome odors from organically overloaded ponds.

In an aerated lagoon, all solids are maintained in suspension.

Aerated lagoons are operated as flow-through activated sludge systems without recycle, usually followed by large settling ponds.

Lagoons and stabilization ponds are currently in use in older plants but are out of favor for new construction due to:

• Odors from algae blooms

• Groundwater contamination concerns because they are only clay lined

• Large land area requirements Seasonal continuous nitrification may be achieved in aerated lagoon systems. The degree of nitrification depends on design and operating conditions within the system and on wastewater temperature. Generally, with higher wastewater temperatures and lower loadings (increased sludge retention time), higher degrees of nitrification can be achieved.

Significant Operational Control Parameters

Aerated lagoons followed by a clarifier with sludge recycle are very similar to activated sludge systems with respect to operation and control.

An aerated lagoon is usually deeper than a facultative pond.

Mechanical or diffused aerators provide most of the dissolved oxy gen required and mix lagoon contents. Turbulence levels should be high enough to ensure uniform dissolved oxygen and suspended sol ids concentrations throughout. The higher the rate of oxygen transfer to the system, the more intense the mixing in the pond, and subsequently, the less pronounced the tendency for sedimentation. Factors that must be considered in the operation of aerated lagoons are similar to those of the activated sludge process and include: biological solids produced, solids retention time, BOD removal, pond mixing, and temperature effects.

Biological Solids

High-rate aerated lagoons can generate 0.6 to 0.7 lb (0.27 to 0.32 kg) of suspended solids per pound (kilogram) of BOD5 removed.

Solids Retention Time

The basis of operation for an aerated lagoon is solids retention time (SRT). Typical SRT design values for aerated lagoons used for treating low-strength industrial wastes vary from 3 to 6 days. Aerated lagoons used in paper mills, refineries, and petrochemical plants have SRT of 10 to 20 days; these generally operate in the extended aeration mode.

BOD Removal

Aerated lagoons that are properly designed operated and maintained, can produce effluent that is low in solids and with effluent BOD5 of less than 30 mg/L. Solids concentrations in effluent are variable (20 to 100 mg/L) and are affected by seasonal changes.

pH and Alkalinity Balance

In general, aerated lagoons operate most satisfactorily in the pH range of 7.2 to 7.8. To achieve desired pH, influent waste pH should be maintained from 6.5 to 8.3.

Oxygen Requirement

Based on operating results obtained from a number of industrial and domestic installations, the amount of oxygen required varies from 0.7 to 1.4 times the amount of BOD5 removed.


The two most important effects of temperature are reduced biological activity and treatment efficiency and formation of ice.

Icing effects may be minimized by increasing the depth of the lagoon or by altering the method of operation. Reducing the area by one-half increases the wastewater temperature. This action corresponds roughly to about a 50% increase in the rate of biological activity.

Stabilization Ponds

A stabilization pond (also called an oxidation pond) is a relatively shallow body of water contained in an earthen basin of controlled shape, designed for treating wastewater. Ponds in the past were used extensively for treatment of industrial wastewater and mixtures of industrial and domestic wastewater that are amenable to biological treatment. Installations are now serving such industries as oil refineries, slaughterhouses, dairies, poultry processing plants, and rendering plants. New regulations, which require double lining of wastewater lagoons that contain specific toxics such as benzene, will limit continued usage of these systems in the future.

Pond Classification

Stabilization ponds are usually classified according to the nature of biological activity taking place: aerobic, anaerobic, or combined aero bio-anaerobic (facultative). Principal types of stabilization ponds in common use are shown in Tbl. 10. Other classification schemes used are based on the type of influent (untreated, screened, settled wastewater, or activated sludge effluent); pond overflow condition (nonexistent, intermittent, or continuous); and method of oxygenation (photosynthesis, atmospheric surface re-aeration, or mechanical aerators).

Stabilization ponds have been used singly or in various combinations to treat both domestic and industrial wastes. Aerobic ponds are used primarily for treatment of soluble organic wastes and effluents from wastewater treatment plants. Aerobic-anaerobic ponds (facultative) are the most common type and have been used to treat domestic wastewater and a wide variety of industrial wastes. Anaerobic ponds are especially effective in bringing about the rapid stabilization of strong organic wastes. Usually, anaerobic ponds are used in series with aerobic-anaerobic ponds to provide complete treatment.

TBL. 10 Types and Applications of Stabilization Ponds

Type of pond or pond system | Common name | Identifying characteristic | Application Aerobic (0.5-2 ft) (0.2-0.6 m) Aerobic (2-5 ft) (0.6-1.5 m) Aerobic anaerobic Aerobic anaerobic T 10 Type

High-rate aerobic pond Low-rate aerobic pond Maturation or tertiary pond Facultative pond Facultative pond with mechanical surface aeration

Designed to maintain aerobic conditions throughout the liquid depth Similar to low-rate aerobic ponds but very lightly loaded Deeper than a high-rate pond.

Photosynthesis and surface reaeration provide oxygen for aerobic stabilization in upper layers. Lower layers are facultative.

Bottom layer of solids undergoes anaerobic digestion.

As above, but small mechanical aerators are used to provide oxygen for aerobic stabilization

Nutrient removal, treatment of soluble organic wastes, conversion of wastes Treatment of soluble organic wastes and secondary effluents Used for polishing (upgrading) effluents from conventional secondary treatment processes, such as trickling filter or activated sludge Treatment of screened or primary settled wastewater and industrial wastes Secondary effluent polishing Treatment of screened or primary settled wastewater and industrial wastes Secondary effluent polishing

Rotating Biological Contactors

A rotating biological contactor (RBC), also known as bio-disc, uses a biological slime of microorganisms, which grow on a series of thin discs mounted side-by-side on a shaft.

This is considered an attached growth biological process. Discs are rotated slowly and are partially submerged in wastewater. Discs are usually made of lightweight plastic. The RBC may be covered to protect the process from low temperatures and to reduce odors. When the process is first started, microbes in the wastewater begin to stick to disc surfaces and grow there until all discs are covered with a 1/16 to 1/8 in (1.6 to 3.2 mm) layer of biological slime. A thin film of wastewater and the organisms on the disc get oxygen from air as the disc rotates. This film of wastewater then mixes with the rest of the wastewater, adding oxygen to the treated and partially treated wastewater. Excess growth of microbes breaks off from the discs and flows to the clarifier to be separated from the wastewater. Rotation of the discs provides fresh media surface for buildup of attached microbial growth, brings growth into contact with wastewater, and aerates wastewater and growths in the wastewater reservoir. The attached growth is like the growth in a trickling filter, except that microbes are passed through wastewater rather than wastewater passing over microbes. The process can achieve secondary effluent quality or bet ter. By placing several sets of discs in series, it is possible to achieve even higher degrees of treatment, including biological conversion of ammonia to nitrate (nitrification).

The RBC system design is based on disc surface area and percent BOD and ammonia removal efficiency. Common loading rates for secondary treatment of municipal wastewaters are 2 to 4 gpd/ft2 [0.08 to 0.16 m3 /(d · m2 )] of effective media area. At temperatures above 59°F (15°C), 90% nitrification can be obtained at loadings of 1.5 gpd/ ft2 [0.06 m3 /(d · m2 )].

ILL. 8 Schematic of RBC process.

ILL. 9 Trickling filter cross section.

Trickling Filters

A trickling filter consists of a bed of coarse material, such as stones, slats, or plastic, over which wastewater is discharged from moving spray distributors or fixed nozzles. A secondary clarifier usually follows a trickling filter to reduce effluent suspended solids. Sometimes, trickling or high-rate filters are used as a first stage treatment for high BOD wastewaters, followed by an activated sludge system. Ill. 9 shows a trickling filter and its principal components, which include:

• Distribution system, which applies wastewater to filter media

• Filter media, which provides surface area for microorganisms to grow

• Underdrain system, which supports the media and provides drainage of waste flow to a collection channel, while permitting air circulation up through the media to supply oxygen to microorganisms The rotary distributor is used to prevent growth of flies that were common in older designs using pipe headers and nozzles. Distributors are rotated by reaction of the discharge of water through orifices on the distributor arms.

Principles of Operation

Trickling filters are not a filtering or straining process as the name implies. The rocks in a rock filter are 1 to 4 in (25 to 102 mm) in diameter, too large to strain solids (Fig. 9). Filters provide large amounts of surface area where microorganisms cling and grow in slime on rocks as they feed on organic matter. Excess growths of microorganisms wash from the rock media and would cause high levels of suspended solids in the plant effluent if not removed. Thus, flow from the filter is passed through a secondary clarifier to allow these solids to settle.

There are several ways to prevent biological slimes from drying out and dying when wastewater flows are too low to keep the filter wet. One method is to recycle filter effluent. Recirculation reduces odor potential and improves filter efficiency as it provides another opportunity for microbes to consume organics that escaped the first pass through the filter. Another approach to improve performance or handle strong wastewaters is to use two filters in series, referred to as a two-stage trickling filter system.

Synthetic media for trickling filters have recently become popular. These materials include modules of corrugated plastic sheets, redwood slats, and plastic rings. These media offer larger surface areas for slime growths, typically 27 ft2 (2.5 m2) surface area per cubic foot (0.028 m3 ) as compared to 12 to 18 ft2 (1.1 to 1.7 m2 ) per cubic foot (0.028 m3) for 3 in (76 mm) rocks, and greatly increase void ratios for increased airflow. The materials are also much lighter than rock (by a factor of about 30), and trickling filters can be much taller without structural problems. While rock in filters is usually not more than 10 ft (3 m) deep, synthetic media depths are often 20 ft (6.1 m) or more, reducing overall space requirements for the trickling portion of the treatment plant. Molded plastic media use pieces of interlocking corrugated sheets of plastic that look like a honeycomb. Sheets are stacked so that they interlock and fit inside the filter structure.

Sludge Reduction Process

Biological wastewater treatment processes always result in generation of a considerable amount of excess sludge that has to be wasted. In general, 0.5 to 0.6 lb (kg) dry sludge is produced when 1 lb (kg) BOD is treated. The expense for excess sludge treatment has been estimated at 40 to 60% of the total expense of wastewater treatment. Moreover, the conventional disposal method of landfilling may cause secondary pollution problems. In areas where landfilling of sludge is restricted, sludge disintegration methods have an economic benefit and fit in the market. However, in areas where sludge disposal options are still relatively inexpensive, sludge disintegration techniques are not readily employed due to additional costs of equipment and operation.

Overall process efficiency can be measured by the ratio of soluble COD divided by total COD. Another way to measure efficiency is to determine the amount of disintegrated solids divided by total sludge solids. At the time of this writing, there are different methods used for this calculation, and there is no apparent universally accepted method as different authors use different calculations.

Sludge reduction can be achieved by inserting sludge disintegration devices in most biological wastewater treatment processes as shown in Fig. 10. In this process, some part of return sludge thickened in the clarifier is sent to a sludge disintegrator to solubilize the sludge. Solubilized sludge that contains soluble BOD and cell debris is sent back to the bioreactors, where live microorganisms consume the BOD and cell debris. Since only about 60% of carbon contained in disintegrated sludge is converted to new microorganisms while about 40% turns to carbon dioxide, net sludge reduction can be readily achieved.

In principle, all kinds of chemical, mechanical, and biological methods that kill microorganisms can be used for sludge disintegration.

While ozone is most commonly used at a dosage of 0.02 to 0.10 g O3 /g TSS, ball mill, microwave, ultrasound, electrolysis, and alkaline/acid treatments can be also used for sludge disintegration. In some commercial processes, biological methods such as thermophilic reactors and anoxic selectors are used for sludge disintegration. In thermophilic processes, part of return sludge is sent to a high-temperature aerobic reactor that is operated at 140 to 158°F (60 to 70°C). In this high temperature condition, the majority of microorganisms contained in return sludge die, and the debris is used to grow thermophilic microorganisms, which die again after recycled to the aeration basin.

An identical logic is used for an anoxic selector.

ILL. 10 Schematic of basic concept of sludge reduction process.

Accumulation of inert materials in sludge can affect overall efficiency significantly. Since about 8% of cell mass is non-biodegradable in aeration basin conditions, inert material can accumulate, if the sludge disintegrator does not convert it to biodegradable materials. In this case, the portion of biologically active solids in MLVSS increases, and the actual F/M ratio goes up. To maintain proper biological activity in sludge, some excess sludge removal is inevitable.. However, if the disintegration process is efficient in converting non-biodegradable materials to biodegradable materials, excess sludge removal can be minimized. In general, chemical sludge disintegration methods have an advantage over mechanical methods in terms of inert material accumulation.

It is known that nitrogen removal is hardly affected by sludge disintegrators, if anoxic reactors are simultaneously used with the aeration basin and disintegrated sludge is supplied to the anoxic tank. It is because the denitrification rate is boosted by additional BOD supplied from the sludge disintegrator. However, phosphorous removal efficiency decreases, since phosphorous removal relies solely on the amount of sludge removal.

Anaerobic Systems

Most of the discussions so far have been about aerobic biological treatment processes. This section addresses anaerobic treatment processes.

Anaerobic Sludge Digestion Anaerobic digestion is one of the oldest processes used for stabilization and reduction of domestic primary and biological sludges.

Typically, it is not used in industrial wastewater treatment systems.

In the process, organic material in sludge is converted biologically to a variety of end products, including methane (CH4 ) and carbon dioxide (CO2), under anaerobic conditions.

The process is carried out in an airtight reactor with sludge introduced and removed on an intermittent basis. There are two basic types of sludge digestion systems: standard rate and high rate. In the standard rate system, reactor contents are unmixed and unheated.

The detention time is 60 to 90 days.

The reactor for the high-rate process is completely mixed and heated to 85 to 100°F (29 to 38°C), resulting in a typical detention time of 15 days or less. The most common configuration is the two-stage system, in which the first stage is heated and mixed. The second stage reactor is not mixed or heated and is allowed to stratify to remove concentrated digested sludge, supernatant liquor, and methane gas. Generally, a scum layer develops on the surface of the liquor in domestic systems. The resulting sludge is well stabilized, and total solids are reduced 45 to 50% by weight. Methane produced is used to heat the digester, heat buildings, generate electricity, or drive mechanical equipment such as pumps and blowers.

Anaerobic Treatment

The anaerobic treatment process involves decomposition of organics in wastewater to methane and carbon dioxide in the absence of oxy gen. Process kinetics and material balances are similar to aerobic systems, but there are certain basic differences that require consideration.

Conversion of organics to methane gas yields little energy, so that the rate of cell growth is slow and the quantity of organic growth is low.

Thus, the rate of treatment and sludge solids yield are both consider ably less than in the activated sludge process.

The anaerobic treatment process is used for treating wastewaters that have high organic content. Among these industries are meat packing, breweries, alcohol production, pharmaceutical, various types of food processing, pulp and paper, and oilfield produced water. Influent BOD and COD in some wastewaters being treated are very high, such as alcohol stillage with 40 g/L BOD and 120 g/L COD. The anaerobic process may be the sole treatment, or it may be ahead of an aerobic treatment system, depending on treated water discharge requirements. Typical organic removals are 75 to 95%, depending on design and application. Tbl. 11 illustrates the BOD/COD characteristics from anaerobic treatment of various waste waters.

Advantages and disadvantages of the anaerobic treatment process as compared to aerobic systems relate directly to the slow growth rate of organisms in anaerobic systems. Slow growth rates mean that reactor detention times have to be relatively long for high efficiency. Slow growth also means that only a small portion of organic material is synthesized into new cells. This means there is a minimal amount of sludge for disposal. A sufficient amount of nutrients must be present, just as in aerobic systems. The amount of nutrients is substantially less than in aerobic systems due to the low growth rate. Most organics are converted to methane that is a useful, combustible product.


Four groups of microorganisms sequentially degrade organic matter in anaerobic fermentation. Hydrolytic microorganisms degrade polymer type material such as polysaccharides and proteins to monomers. This reaction results in no COD reduction.

Monomers are then converted into fatty acids by acid-forming bacteria with a small amount of hydrogen (H2 ). Principal acids are acetic, propionic, and butyric. In the acidification stage, there is mini mal reduction of COD. If a large amount of H2 occurs, some COD reduction occurs. This seldom exceeds 10%.

Wastewater source BOD, mg/L COD, mg/L

Sugar 50-500 250-1500 Dairy 150-500 250-1200 Maize starch - 500-1500 Potato 200-300 250-1500 Vegetable. 100 700 Wine 3500 - Pulp 350-900 1400-8000 Fiber board 2500-5500 8800-14 900 Paper mill 100-200 280-300 Landfill leachate - 500-4000 Brewery - 200-350 Distillery - 320-400

Tbl. 11 BOD and COD in Anaerobic Treatment Effluents

All acids higher than acetic acid are converted to acetic and H2 by acetogenic microorganisms. The conversion of propionic acid is:

CHO2HOCHOCO3H 362 2 242 2 2 +? ++

In this reaction, COD reduction does occur in the form of H2.

Acetic acid and H2 are converted to methane by methanogenic organisms (methane formers):

Acetic acid:

CHO CO CH 242 2 4 ?+

CH COO H O CH HCO 34 -- +?+ 23


HCO H CH OH H O 2 32 4 42 -- +?+ +

TBL. Typical Coefficient Values for Monod Relationship:

Temperature, deg F (deg C) Kmax , d-1 KS, mg/L 95 (35) 6.67 164 77 (25) 4.65 930 68 (20) 3.85 2130

The kinetic relationship commonly employed for anaerobic degradation is the Monod relationship:

ds dt //() max =+ KSXKS S (22) where ds/dt = substrate (COD) utilization rate, mg/(L · d)

K_max = maximum specific substrate utilization rate, g COD/ (g VSS · d)

S = effluent COD concentration, mg/L

X = biomass concentration, mg/L

KS = half saturation concentration, mg/L

One pound (0.45 kg) of COD or ultimate BODL removed in the process yields 5.62 ft3 (0.16 m3) of methane at standard conditions [32°F (0°C) and one atmosphere] and 6.3 ft3 (0.18 m3) at 35°F (1.7°C). Low BOD and COD wastewaters may not produce enough gas to heat the reactor. Conversely, high strength wastes produce excess gas that is used as a source of energy such as heat, electricity, and the like.

Coefficients for Eq. are shown in Tbl..

Actual organic loading to the reactor depends on reactor design and type of waste. Bench and pilot tests have to be conducted, if the supplier of the anaerobic system has not had prior experience on the type of waste. Startup of an anaerobic system can take anywhere from a few months to nine months or more, depending again on the type of organic waste. Seed sludge taken from a similar application reduces startup time.

Membrane Bioreactors

Use of membrane bioreactors (MBR) has expanded considerably, from a few systems in the 1980s to several thousands in 2005. In these systems, ultra-filtration (UF) or microfiltration (MF) membranes replace sedimentation basins for separation of biomass from water.

MBR Process

The membrane can be installed in the bioreactor tank or in an external tank, where biomass is continuously separated from water. Since solids-liquid separation is performed by a membrane, lower effluent TSS can be achieved compared to a conventional settling basin. More importantly, MBR decouples the biological process from the process of settling biomass. This allows operation of the biological process at conditions that would be untenable in a conventional system, including high MLSS of 8 to 15 g/L, long sludge retention time, and low hydraulic retention time. In conventional systems, such conditions could lead to sludge bulking and poor settle-ability.

The MBR process replaces the conventional clarifier with membranes, which can be submerged directly in the aeration basin or reside in an external tank. These microporous membranes have a pore size range between 0.04 to 0.4 µm and allow for almost complete particle separation from mixed liquor. MBR has the following benefits:

• MLSS can be increased to 8 to 15 g/L (versus 1.5 to 8 g/L for conventional systems)

• Higher MLSS can reduce HRT

• Lower sludge production

• Lower effluent TSS

• Lower effluent BOD

• Up to 50% footprint reduction

• Higher SRT can produce good nitrification

A conventional system is shown in Fig. 11, while the MBR system is shown in Fig. 12. Membranes serve as the separation device for solids-liquid separation instead of a clarifier or dissolved air flotation (DAF) unit. Membranes are used in this submerged configuration and operated under vacuum, or they are used external to the aeration basin and operated under pressure. In a submerged MBR (sMBR), a suction pump is used to pull clean water through the membrane, while excluding passage of particles. In an external MBR (eMBR), a recirculating pump is used to deliver mixed liquor under pressure to the membranes and then back to the aeration basin.

In sMBR, air bubbles are released directly underneath the membranes, and this provides the shear forces necessary to minimize solid particle build up on the membrane surface. The main operating variables used to maintain flow across the membrane surface (flux) are the aeration rate and suction pressure, which is the driving force that controls the amount of water transported across the membranes, and is directly related to flux. Crossflow velocity is the main operating variable that controls flux in eMBR.

ILL. 11 Conventional activated sludge process.

ILL. 12 Membrane bioreactor (MBR) process.

ILL. 13 Hollow fiber MBR membrane configuration.

ILL. 14 Flat sheet membrane configuration.

In addition, to further maintain flux, both systems depend on intermittent operation. During the off cycle, the membrane surface continues to be scoured by water and air, and solid debris is loosened and removed. In some membrane systems using hollow fibers, a periodic back pulse of permeate is used to help remove accumulated sol ids. Membranes can be taken out of service and cleaned chemically with sodium hypochlorite, caustic, or organic acids.

Membranes are constructed of polymeric or ceramic materials.

The vast majority of membrane systems are made from polymeric materials such as poly-vinylidine difluoride, polyethylene, and chlorinated polyethylene. Membrane geometry is usually either hollow fiber or flat sheet.

In hollow fiber and flat sheet membranes, water flow is from the outside to the inside. Pore openings can range from the ultrafiltration to the microfiltration size. Key operational variables are flux, trans membrane pressure (TMP), and membrane aeration rate.

Gross flux is calculated from Eq. (23):

Flux G = FA / (23) where Flux G = gross flux, gal/[h · ft2 ] [L/(h · m2 )]

F = flow rate, gph (L/h)

A = membrane area, ft2 (m2 ) The net flux is the gross flux minus the time required for relaxation, during which there is no water flow and is given by Eq. (24):


Flux Relax NG =- (%)/ 100 x 100 where Flux N is the net flux, gal/[h · ft2 ] [L/(h · m2 )] and Relax% is the time required for relaxation, %.

MBR Limitations

Use of MBR is growing worldwide at a rate of 25 to 30% per year.

Driving this growth rate are the many advantages of MBR over conventional wastewater treatment processes. However, membrane fouling is the one problem that is limiting even greater expansion, and is causing many operational issues at existing MBR plants. Various approaches have been tried to reduce membrane fouling, such as intermittent suction, back flushing, module design improvement, and optimization of aeration. Combinations of these methods have reduced costs of MBR significantly, but further improvements are possible.

Membrane fouling is directly related to condition of the biomass.

Although MBR does not have to operate under conditions to form settleable floc such as in a waste activated sludge system, certain operating conditions tend to generate more foulants and make membranes more susceptible to fouling and flux loss. Reasons why these foulants form are complex and not completely understood. However, frequent changes in influent feedwater quality, lack of equalization, frequent peak flow events, insufficient dissolved oxygen, and poor control of MLSS concentration are contributing factors that tend to generate more foulants and increase membrane fouling. These factors are all inter linked to the operating parameters of the MBR such as HRT, SRT, and percent MLSS. The main objective of any MBR is to obtain long-term sustainable flux and good water quality under normal and peak flow conditions, and to accomplish this at minimum cost.

Microbial Foulants

Soluble microbial products (SMP) are major membrane foulants in the MBR processes. SMP consist of soluble polysaccharides and protein biopolymers that are produced by microbial organisms. This material is released when microbial cells lyse. SMP material along with other submicron particles can deposit on membrane surfaces and restrict flow of water through the membrane. If particles are smaller than the membrane pore size, this material can block the pores and irreversibly foul the membrane surface. Irreversible foulants cannot be removed by the techniques described above, but instead, membranes must be taken off-line and cleaned by submersion and soaking in various cleaning solutions, such as bleach, acids, and caustic. Even this aggressive cleaning sometimes fails to remove foulants. Irreversible fouling is the most significant contributor to reduced membrane life.

SMP material that is larger than the membrane pore size can coat the surface of the membrane and form a gel layer. This layer can actually help prevent particles that are smaller than membrane pores such as viruses from passing through the membrane into the effluent water. However, if this gel layer becomes too thick, there is often increased resistance that reduces membrane flux.

After membranes are cleaned using cleaning methods recommended by the supplier of the membranes, filtration rate and membrane pump pressures are monitored to determine the extent of membrane filtration improvement. The cleaning process should have brought the membrane performance back to design conditions. If not, then significant fouling has occurred, and either additional cleaning or replacement of membranes could be required.

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