Wastewater Treatment

Wastewater Treatment

[T1] Wastewater – title page

[W] Backstory (11.0)

1. Protection of Receiving Waters

Despite the fact that raw wastewater is considered grossly polluted, the amount of contaminants which it contains is rather small: a cubic meter of wastewater weighs approximately 1,000,000 grams and has 500 grams of pollutants ... that’s five one-hundredths of 1%! Yet this small fraction can have serious impacts if discharged to a lake or river.

[W] Introduction (11.1)

The purpose of municipal wastewater treatment is to prevent pollution of the receiving watercourse. Examples of receiving water pollution resulting from wastewater discharges include dissolved oxygen depletion (BOD), eutrophication (N and P), toxicity (NH3, metals, organics), and pathogens (bacteria and viruses). Here we will examine the sources and nature of wastewater and explore the unit operations necessary to yield an effluent in compliance with state and federal regulatory requirements.

[T2] The regulatory basis

Through the Federal Water Pollution Control Act Amendments of 1972, commonly known as the Clean Water Act, Congress established a national strategy to reduce water pollution. The objectives of the Act were to “restore and maintain the chemical, physical, and biological integrity of the nation’s waters” by achieving a level of water quality which “provides for the protection and propagation of fish, shellfish, and wildlife” and “for recreation in and on the water” [fishable/swimmable] and to eventually “eliminate the discharge of pollutants into United States waters” [zero-discharge].

The Clean Water Act establishes required levels of treatment technologyfor point source discharges, (technology-based effluent limits; e.g. secondary treatment for municipal wastewater treatment plants). Where water quality remains impaired following implementation of technology-based standards, total maximum daily loads (TMDLs) are established to insure that standards are met (water quality-based effluent limits). Effluent limits for point source pollutant discharges to surface waters are applied and regulated under the National Pollutant Discharge Elimination System (NPDES). Permit violations may result in civil (fines) and criminal (prison) penalties.

[T3] Modeling receiving water impacts

[T4] NPDES permit system

[T5] Treatment plant design

2. Wastewater Sources

[W] Wastewater sources (11.2)

Municipal wastewater treatment plants or publicly-owned treatment works (POTWs) receive inputs from domestic and industrial sources. The components of a domestic wastewater are:

wastewater from homes and commercial premises

infiltration

inflow

Infiltration is the water entering a sewer system from the ground through such means as defective pipes, pipe joints, connections, and manhole walls. Inflow includes steady sources (i.e. water discharged from cellar and foundation drains, cooling water, and drains from springs and swampy areas) and direct sources (i.e. roof leaders and yard drains, manhole covers, and combined sewers) related to stormwater runoff. Infiltration and inflow are problems in that they add to the total volume of wastewater that must be treated and, by dilution, influence its character. Infiltration and inflow can have a negative impact on waste treatment systems by:

overloading collection systems leading to backups and overflow

reducing the efficiency of treatment (i.e. strength, retention time)

Thus most municipalities have programs to reduce infiltration and have banned the direct hookups that lead to many forms of inflow, i.e. roof drains.

Industrial wastewaters vary widely in quantity, composition, and strength depending on the industry by which they are generated.

[T6] Industrial wastewater sources

While industrial wastes may contain some of the pollutants found in domestic wastewater, they may also contain chemicals such as heavy metals, organic chemicals, and radioactive substances which:

interfere with plant operation
‘pass through’ the plant untreated
limit opportunities for recycle of effluent and sludge

U.S. EPA has identified 126priority pollutants based on their potential as carcinogens, mutagens, teratogens, or acute toxicity.

[T7] Some priority pollutants

3. Domestic Wastewater Characteristics

Fresh, aerobic domestic wastewater is a gray, turbid liquid with a temperature of 10-20 C and an odor of kerosene or freshly turned earth. Aged septic sewage is black and has the characteristic rotten egg odor of hydrogen sulfide.

[W] Sooch (11.3)

[T8] What’s In Wastewater? – Waste

[T9] What’s In Wastewater? - Water

[T10] It’s Mostly Water

[T11] So What’s the Problem?

The most common contaminants present in domestic wastewater are:

Pathogens (bacteria, protozoa and viruses)
Solids
Nutrients (nitrogen and phosphorus)
Organic matter (oxygen-demanding materials)
Toxics (metals and synthetic organic chemicals)

[T12] Pollutant concentrations in raw domestic wastewater

[W] Wastewater treatment overview (11.1)

4. Wastewater Collection Systems

Wastewater is discharged from homes, commercial establishments, and industrial sites via sanitary sewers, i.e. pipes flowing partially full and not under pressure. The connection between houses or other buildings and the main sewer is made with 6 inch diameter or larger pipe. Collecting sewers (8-12 inch diameter) gather flows from individual buildings and transport the material to an interceptor or trunk sewer (15-27 inches in diameter) which delivers the flow to the wastewater treatment plant. Pipe sizes are provided here as examples: as one moves from the perimeter of cities toward the treatment plant, trunk sewers may reach a diameter of several meters.

[T13] Where Does It Go When I Flush the Toilet

[T14] Collection system (Clark et al., p. 209)

Sanitary sewers are constructed of concrete pipe placed 1-3 meters below the ground surface to avoid freezing. The sewers are designed to flow by gravity at a rate which will:

prevent in-line sedimentation of solids

deliver the wastewater to the plant before anaerobic conditions set in

avoid scour of the pipe material

Sometimes, due to topography, it is impossible to install all gravity sewers. Wastewater is then accumulated at a low point in the collection system and pumped to the treatment plant or to a continuation of the sewerage system at a higher elevation. Pumping stations consist of a wet well, which intercepts incoming flow and permits equalization of pump loadings, and a bank of pumps which lift the wastewater from the well to force mains or pressurized pipes.

[T15] Portage transmission system (travel time 2.5 hrs from Houghton; add 2 hours from Hancock)

Today, most sewer systems are designed to transport only wastewater. In older cities, combined sewers which transport both wastewater and stormwater have relief structures (combined sewer overflows or CSOs) which discharge a raw sewage - stormwater mixture to receiving waters under high flow conditions.

[W] Combined and separate sewer systems (9.4)

There are no combined sewers in the Portage system; all the CSOs have been sealed. Problems remain with infiltration and inflow which amounts to 30-40%, especially during spring runoff.

5. Variability in Wastewater Flow

Knowledge of rates of wastewater flow is required in the hydraulic and process design of a wastewater treatment plant. Hydraulic design seeks to minimize overload problems (backups; flooding), while process design seeks to avoid inefficient operation (e.g. insufficient substrate or insufficient retention). The design must accommodate the variation in domestic wastewater flow rates and the associated waste load (BOD, SS) which occurs over the day.

[T16] Variation in wastewater flow (M&E, p. 180)

Flow rates are low after midnight when little domestic wastewater is discharged. At that time, a substantial part of the dry weather flow is infiltration and sewage strength is weak. Flow rates increase in the morning as water demand rises and peak again in the evening at bedtime. There is also a dramatic difference in wastewater flows in dry versus wet periods.

[T17] Dry/Wet period wastewater flows (M&E, p. 164)

Industrial wastewaters may be generated on a continuous (i.e. steel industry) or batch (i.e. chemical and pharmaceutical industries) basis. Each industry has its particular characteristics and a waste survey is required to determine the flow and waste load.

Design flows are best determined from field measurements of wastewater flows. Where actual flow rates are not available, rates may be estimated from water use records (60-85% of the per capita consumption of water becomes wastewater). For new communities, design flows can be calculated based on the design population and projected water consumption for domestic use and commercial and industrial activity. Flow equalization techniques (discussed later in relation to preliminary treatment) may be applied where the process design cannot efficiently be sized to accommodate maximum flows.

6. Preliminary Treatment

Preliminary treatment prepares a wastewater for further treatment by conventional primary (gravity settling) and secondary (biological) processes. For municipal wastewaters, preliminary treatment is used to remove oily scum, floating debris and grit which may inhibit biological processes and/or damage mechanical equipment. Chemicals may also be added to neutralize acids or bases originating from industrial sources. Equalization tanks are employed to balance flows or organic loading. Industrial effluents may additionally require physical-chemical pre-treatment for removal of ammonia-nitrogen (air-stripping), heavy metals (oxidation/reduction, precipitation) or oils (dissolved air floatation).

a. Screening:

Bar racks (parallel bars or rods) or screens (perforated plates or mesh) retain the coarse solids (rags, plastic bottles, etc.) present in wastewater, preventing damage to mechanical equipment. Bar racks and screens are cleaned by hand in some older, smaller plants, but most are equipped with automatic cleaning rakes. Screenings are typically disposed of at landfills or by incineration.

[T18] Preliminary treatment –bar rack

[T19] Preliminary treatment - screen

In some plants, the next treatment step is a comminutor; a grinder which cuts up (comminutes) the solids that pass through bar racks and screens without removing them from the wastewater flow. This reduction in size makes the solids easier to treat in subsequent operations.

[T20] Preliminary treatment - comminutor

Comminutors may also be used as a low-maintenance alternative to screens, eliminating the need for screenings handling and disposal. Some engineers believe that it is inappropriate to return solids to the waste stream once they have been removed and others counsel against using comminutors because plastic pieces can inhibit biological treatment.

b. Grit Chambers

Grit is inorganic material (e.g. sand, ~1 mm) which is washed into the sewer system from roadways and pavement. Although our local sewers are almost completely separated, some grit still gets in from manhole covers and car washing operations, etc. Grit has the potential to abrade mechanical equipment, but more importantly, it settles out in subsequent operations. Since grit doesn’t require additional treatment, it is best to get it out of the waste stream early. Grit is typically removed in horizontal grit chambers, aerated to keep the less dense organic materials in suspension and to freshen the sewage, making it easier to treat in subsequent steps.

[T21] Preliminary treatment - grit chambers

Because the lighter organic solids are segregated from the grit, residue from the grit chamber may be washed and sent to landfills with little concern for odor or other problems. Grit is composed of discrete particles, whose settling behavior is well-described by Stokes’ Law.

Example Problem: Grit Chamber Design

c. Flow Equalization

The flow rate (hydraulic load) and strength (organic load) of a wastewater influent vary considerably over the day. Industrial discharges may impart additional imbalances creating conditions of nutrient-deficiency or toxicity. The constantly changing amount and strength of wastewater to be treated makes efficient process operation difficult. Equalization may typically be required to accommodate variation in:

 floworganic load

nutrients pH

The purpose of flow and organic loading equalization is to dampen variations so that the wastewater can be treated at a nearly constant flow rate and strength. Flow equalization is achieved by constructing large basins near the head of the treatment works, but downstream of other pre-treatment facilities. Adequate aeration and mixing must be provided to prevent odors and solids deposition. Wastewater is continuously collected in these basins and pumped for treatment at a constant rate. Nutrients may be added to nutrient-deficient influents and acids or bases added to adjust the influent pH to levels appropriate for biological treatment. The impact of high toxicity loads may be also be reduced through balancing.

[T22] Preliminary treatment – process summary

[W] Preliminary treatment - process summary (11.4)

7. Primary Treatment

The goal of primary treatment is to remove solids through quiescent, gravity settling. Wastewater is held in circular or rectangular basins referred to as settling tanks, sedimentation tanks or clarifiers.

[P] Primary treatment - rectangular settling tank

[P] Primary treatment - circular settling tank

Solids settle to the bottom of the tank where they are collected as a liquid-solid sludge. Floating matter and grease are collected from the surface by skimming. The process yields a partially clarified effluent and a liquid-solid sludge.

The solids collected in primary clarifiers do not settle as discrete solids (Stokes’ Law), but rather stick to adjacent particles, growing in size as they settle (Type II Sedimentation or flocculent settling). Sedimentation tanks are normally designed on the basis of the surface overflow rate (SOR, m3m-2d-1). Settling tests are conducted in the laboratory, yielding a relationship between the SOR and the removal of suspended solids and BOD.

[P] Design - performance relationship for primary treatment

The SOR corresponding to the desired removal efficiency is identified from the figures and the required tank area is calculated. The volume of the tank is then determined as the product of the design surface area and the tank depth, typically 3-4m. Detention times can be calculated based on inflow and tank volume and typically range from 1-2 hours.

Example Problem: Primary Treatment Design

Primary treatment removes about 60% of the suspended solids, 30% of the BOD, and 20% of the phosphorus contained in the influent. Higher removal rates may be achieved by adding coagulant aids, i.e. chemicals such as iron salts, aluminum sulfate (alum) or calcium hydroxide (lime) which enhance settling. The clarified effluent produced through primary treatment is routed to the next stage of treatment and the sludge is segregated for further treatment and ultimate disposal. Primary sludge is putrescible, may contain pathogenic organisms, and has a high water content; all characteristics which make disposal difficult.

8. Secondary Treatment

The water leaving the primary clarifier has lost much of the solid matter it contained, but still has a high demand for oxygen due to an abundance of dissolved organics matter (BOD). Secondary or biological treatment utilizes microbial action to decompose these energy-rich molecules.

[P] Biochemistry of biological treatment

[P] The microbial loop in nature

[P] Wastewater treatment microorganisms

[P] The microbial loop in wastewater

There are two basic approaches to biological treatment, differing in the manner in which the waste is brought into contact with the microorganisms. In suspended growth reactors, the organisms and waste material are mixed together, while in fixed film reactors, the organisms are held in place and the waste stream is passed by.

Fixed Film Reactors

Trickling filters provided the first successful application of microorganisms in waste treatment.Here, primary effluent is ‘trickled’ over and percolates through a tank up to 60 m in diameter filled with fist-sized rocks (1-3 m deep) or plastic media (up to 12 m deep). Abiological film, up to 10 mm in thickness, forms on the solid surfaces, extracting organic matter (BOD) from the waste as food as the waste trickles by. The waste is then degraded under aerobic conditions in the first 0.1-0.2 mm and under anaerobic conditions in the remainder. As the film grows in thickness, food resources are depleted before they reach the innermost parts of the film, the microbes die and the film is lost through sloughing. The microbial biomass lost through sloughing is removed from the waste stream in a secondary clarifier.

[P] Trickling filter

Trickling filterdesign is based on maximum allowable hydraulic (4-10 m3m-2d-1) and organic (240-480 gBODm3d-1) loads. The organic load must be limited to avoid food saturation of the microbial population (poor removal efficiency) and the hydraulic load must be controlled to avoid drying or ponding (inadequate oxygen).

Example Problem: Trickling Filter Design

Another type of fixed film reactor is the rotating biological disk where microorganisms grow on the surface of large disks which are immersed in and slowly turn through the waste. Organic matter is absorbed when the disks pass through the waste and air is taken up when the disk surface is out of the waste tank.

[P] Rotating biological disks

Suspended Growth Reactors

The most commonly applied biological treatment system is a suspended growth approach called the activated sludge process. Effluent from the primary clarifier is introduced to an aeration tank and mixed with a mass of microrganisms composed of bacteria, fungi, protozoa and rotifers. This mixture of liquid, waste solids, and microorganisms is called the mixed liquor suspended solids (MLSS, mgL-1). The organisms absorb dissolved organics and break them down into carbon dioxide, water, and some stable compounds. Bacteria are primarily responsible for assimilating the organic matter in wastewater and the rotifers and protozoa are helpful in removing the dispersed bacteria which otherwise would not settle out. The energy derived from the decomposition process is used for cell maintenance and to produce more microorganisms. Once most of the dissolved organics have been used up, the MLSS is routed to the secondary (or final) clarifier where the microorganisms are separated from the clean water. As with primary settling, two streams are produced: a clarified effluent which is sent to the next stage of treatment and a liquid-sludge composed largely of microorganisms. Lying at the bottom of the final clarifier, without a food source, these organisms become nutrient-starved or ‘activated’. A portion of the sludge is then pumped to the head of the tank (return activated sludge) where the process starts all over again. The remainder of the sludge is processed for disposal (waste activated sludge). It is necessary to continuously waste sludge to balance the gain through microbial growth.