Physical and biological treatment of sewage lecture 1 of 2

PHYSICAL AND BIOLOGICAL TREATMENT OF SEWAGE

CHAKAMBAJ

INTRODUCTION

• Sewage is a major carrier of disease (from human wastes) and toxins (from industrial wastes).

• The safe treatment of sewage is thus crucial to the health of any community.

• This article focuses on the complex physical and biological treatments used to render sewage both biologically and chemically harmless.

• The wastewater discussed in this section is predominantly of domestic origin.

• Varying amounts of industrial and laboratory wastewaters can be collected and treated with the sanitary sewage.

• The primary purpose of the treatment of sewage is to prevent the pollution of the receiving waters.

• Many techniques have been devised to accomplish this aim for both small and large quantities of sewage.

SEWAGE TREATMENT

• Sewage is a mixture of domestic and industrial wastes. It is more than 99% water, but the

remainder contains some ions, suspended solids and harmful bacteria that must be removed before the water is released into the sea.

• The treatment of wastewater is divided into three phases: pre-treatment, primary treatment and secondary treatment.

Pre-treatment

• Large solids (i.e. those with a diameter of more than 2cm) and grit (heavy solids) are removed by screening.

• These are disposed of in landfills.

Primary treatment

• Primary treatment removes most of the solids from the effluent, but doesn't remove or degrade the dissolved organic matter

• The water is left to stand so that solids can sink to the bottom and oil and grease can rise to the surface.

• The solids are scraped off the bottom and the scum is washed off with water jets. These two substances are combined to form sludge.

Secondary treatment

• Secondary treatment uses micro-organisms to convert these organics to simple compounds, and uses the energy of the sun to destroy pathogens.

• The effluent is then safe to be discharged into the water body. The entire process is shown

• diagrammatically in Figure 1.

THE SEWAGE COLLECTION SYSTEM

COLLECTION SYSTEM

• The purpose of a sewage collection system is to remove wastewater from points of origin to a treatment facility or place of disposal.

• The collection system consists of the sewers (pipes and conduits) and plumbing necessary to convey sewage from the point(s) of origin to the treatment system or place of disposal.

• It is necessary that the collection system be designed so that the sewage will reach the treatment system as soon as possible after entering the sewer.

• If the length of time in the sewers is too long, the sewage will be anaerobic when it reaches the treatment facilities.

• Sanitary sewage collection systems should be designed to remove domestic sewage only.

• Surface drainage is excluded to avoid constructing large sewers and treating large volumes of sewage diluted by rainwater during storms.

• Sewers which exclude surface drainage are called sanitary sewers, and those which collect surface drainage in combination with sanitary sewage are called combined sewers.

• Sewers are laid to permit gravity flow of their contents.

• Unlike water in a water distribution system, the contents of a sewer do not flow under pressure.

• The gradient must not be too steep or to gentle

• This is a self-cleansing velocity and prevents solids from settling in the sewer pipes. To the maximum extent practical, sewers are laid in straight lines.

• Corners and sharp bends slow the flow rate, permit clogging, and make line cleaning difficult.

• Pumping is necessary where the slope of the sewer does not produce the required minimum velocity or where sewage must be lifted to a higher elevation.

• Sewage can be pumped from pumping stations through pressure lines (force mains) regardless of their slope, or it can be raised to a higher elevation at pumping stations (lift stations), so that gravity flow will again produce the required velocity.

• For gravity flow lines, sewer pipes of vitrified clay tile, concrete, cement-asbestos, or bituminous-impregnated fibre may be used.

• For force mains and stream crossings, cast iron or cement-asbestos pipes are used.

• Removing grease from sewage is essential to the proper functioning of sewage systems.

• At fixed installations, grease is collected by ceramic or cast iron grease interceptors installed at kitchens and other facilities that generate grease and by concrete or brick grease traps outside the building.

• Approximately 90 per cent of the grease will be removed from greasy wastes by properly maintained grease interceptors and traps.

• On arrival at the treatment works sewage is soup like, dirty liquid containing food, detergents, human waste, oils, sand and sometimes harmful chemicals such as acids.

• All these pollutants have to be removed and the water cleaned before it can be returned to the natural water cycle.

• The sewage treatment process can be complex. However, the key elements of the process are:

Pre-treatment

• Pre-treatment removes the large solids (such as rags and sticks) that are carried in with the wastewater.

• These are removed by screens consisting of metal bars spaced at 19 mm intervals which are placed across the influent channels.

• Tines (metal combs) rake the collected matter off these, and heavy objects such as rocks (which would otherwise damage the equipment) are then collected for land filling off site

2. Grit and Sand Channels• The sewage then flows along wide, deep

channels where grit and sand sink to the bottom.

3. Sedimentation Tank• Here, solid particles in the sewage settle at the

bottom of the tank to form a thick sludge.

4. Filtration

A. Biological Filter• The ‘settled sewage’ is then sprinkled onto large circular beds,

about two metres deep, filled with stones or clinker. Biological filtration is based on the principle that where enough air is present cultures of bacteria will form.

• Millions of bacteria and other tiny creatures live on the stones and feed on the organic material in the sewage, converting it into carbon dioxide, water and nitrogen compounds.

• They literally ‘eat’ the sewage, removing harmful waste. This biological activity produces a humus sludge which settles out in special humus tanks.

• Activated Sludge• In this alternative to biological filtration,

activated sludge containing bacteria is mixed with the settled sewage in an aeration tank.

• The air the bacteria need is provided by water in the tank.

5. Final Settling Tank

• Small and fine particles settle out leaving the cleaned water, or effluent, to flow back into rivers and streams.

6. Sludge

• Sludge can be defined as the solid waste which has settled out of the sewage in the sedimentation, humus and final settling tanks.

• tonnes of sludge are produced every year and several methods of disposal are used:

a) Sludge contains nitrogen, phosphorus and organic matter which makes it ideal for use as an agricultural fertiliser. 68 per cent is used in this way.

b) Sludge from sewage works in heavy industrial areas may have a high metallic content and therefore cannot be used on the land.

• In these cases the sludge can be dried into a cake and used for landfill purposes and tipping, or it can be incinerated.

The role of the laboratory

• A wide variety of analytical tests are used to determine the purity of the wastewater at various stages of treatment so that the possibility of harm to either people or the environment is minimised.

Sewage disposal

A detailed account

Pre-treatment

• Large solids (i.e. those with a diameter of more than 2cm) and grit (heavy solids) are removed by screening.

• These are disposed of in landfills.

Primary treatment

• Primary treatment removes most of the solids from the effluent, but doesn't remove or degrade the dissolved organic matter

• The water is left to stand so that solids can sink to the bottom and oil and grease can rise to the surface.

• The solids are scraped off the bottom and the scum is washed off with water jets. These two substances are combined to form sludge.

Secondary treatment

• Secondary treatment uses micro-organisms to convert these organics to simple compounds, and uses the energy of the sun to destroy pathogens.

• The effluent is then safe to be discharged into the water body. The entire process is shown

• diagrammatically in Figure 1.

• The sludge is further treated in 'sludge digesters': large heated tanks in which its chemical decomposition is catalysed by microorganisms.

• The sludge is largely converted to 'biogas', a mixture of CH4 and CO2, which is used to generate electricity for the plant.

• The liquid is treated by bacteria which break down the organic matter remaining in solution.

• It is then sent to oxidation ponds where heterotrophic bacteria continue the breakdown of the organics and solar UV light destroys the harmful bacteria.

Oxidation

• Conditions in the ponds promote the growth of unicellular algae: minute plants which, like any other plants, absorb carbon dioxide in daylight and give off oxygen by photosynthesis.

• This oxygen oxidises the organics, thus purifying the sewage by reducing its oxygen demand.

• The ponds absorb an amount of solar energy equivalent to a 745 kW engine running continuously.

• Such engines are in fact required by other modern sewage treatment processes where the works must be restricted to a smaller area.

The role of the laboratory

• A wide variety of analytical tests are used to determine the purity of the wastewater at various stages of treatment so that the possibility of harm to either people or the environment is minimised.

• Step 1 - Pre-treatment• Pre-treatment removes the large solids (such as rags

and sticks) that are carried in with the wastewater.• These are removed by screens consisting of metal

bars spaced at 19 mm intervals which are placed across the influent channels.

• Tines (metal combs) rake the collected matter off these, and heavy objects such as rocks (which would otherwise damage the equipment) are then collected for land filling off site

• Step 2 - Primary Treatment• Here grit (fine, hard solids), suspended solids and scum are removed in two stages.• Preaeration• Firstly the wastewater is aerated by air pumped through perforated pipes near the

floor of the• tanks. This aeration makes the water less dense, causing the grit to settle out. As the

air jets are• positioned such that the water is swirling as it moves down the tanks the suspended

solids are• prevented from settling out. The air also provides dissolved oxygen for the bacteria to

use later• in the process, but the wastewater is not in these tanks long enough for bacterial

action to occur• here. The grit is collected in hoppers and washed, after which it is used for on site land• reclamation and landscaping.

• Sedimentation• The water then flows slowly and smoothly through the

sedimentation tanks, where the suspended• solids fall to the bottom and scum rises to the surface,

while clarified effluent passes on. The• solids are removed from the bottom of the tanks by

scrapers, and scum is washed off with water• jets. The scum and solids are brought to a common

collection point where they are combined to• form 'sludge' and sent off for secondary treatment.

• Step 3 - Secondary Treatment• After secondary treatment all effluent, both solid and liquid, is sufficiently safe to be released• into the environment. The treatment of solids and liquids are covered separately below.• Solids• Sludge from the sedimentation tanks is digested anaerobically in large tanks, and then further• digested in lagoons before being dried in dewatering beds. In the sludge digesters the sludge is• kept at 37oC and mechanically mixed to ensure optimum operation. During this time the organic• compounds within the sludge are converted to carboxylic acids and then finally to methane and• carbon dioxide. This gaseous mix is known as "biogas", and is a valuable source of fuel. At• Mangere it is used to generate electricity, which is primarily used to drive the plant machinery,• with any excess electricity being sold to Mercury Energy. The exhaust created by burning the• biogas is used to heat the sludge in the digesters.

• When the sludge leaves the digesters it has undergone a 50% volume reduction. It is then sent to

• lagoons for about a year, and finally to dewatering beds. During this time all pathogens are

• killed by the sunlight.• A small proportion of the sludge is currently mechanically dewatered instead of being

treated in• the lagoons and dewatering beds. Polyelectrolytes are added to the sludge, and the

attraction of• opposite charges causes a floc (loose aggregation of particles) to form. Most of the

water is then• removed by rollers squeezing the mixture. After this the sludge has become

concentrated from 4• to 30% solids, and could potentially be used as a soil conditioner, although currently it

is simply• landfilled.

• Liquids• The liquids are either sent directly to open-air oxidation ponds, or sent to 'fixed growth

reactors'• to reduce their BOD before pond oxidation. The fixed growth reactors (FGR's) are tall,

circular• tanks covered with fibreglass. Each one is filled with approximately 36 million 10 cm

diameter• PVC 'wheels'. Microorganisms live on the wheels, and these reduce the BOD of the sewage

by• a further 75%. The wastewater is sprayed on the top of the wheels and percolates down,

with the• organics being reduced to CO2, CH4 and a small amount of foul-smelling H2S. From the

bottom• of the FGR the effluent is piped to one of the secondary sedimentation tanks where sludge• (consisting mainly of dead microorganisms from the FGR's) is removed. This sludge is piped• back to join the incoming wastewater and complete the cycle again.

• The wastewater then joins the liquid that was sent straight to the ponds for oxidation, where an

• influent mix of primary and secondary treated wastewater and some recycled pond water is

• received. The mix is precisely calculated to ensure that the amount of organics entering the pond

• is the optimum amount for the bacteria to process given the amount of oxygen available at that time.

algae

O2

bacteria

CO2

N2

H2S

CH4

NH3

• A diagram of the processes taking place in the ponds is given in Figure 2. The effluent entering

• the ponds is a mixture of primary and secondary treated effluent, with the relative proportions

• varying during the year. During summer the pond algae produce large amounts of O2 by

• photosynthesis, so the proportion of effluent that has only been primary treated (i.e. the

• proportion that requires large amounts of O2) is greater, whereas in winter more secondary

• treated effluent is used.

• THE ROLE OF THE LABORATORY• The laboratory carries out a significant range of tests at various stages of the treatment

process to• ensure effluent purity and to protect water and equipment at other stages of the process.

Many of• these tests can only be used to monitor a single parameter, but a small number have much

wider• application. These are:• • Atomic absorption spectroscopy (AA), which is used to detect the presence of many• metals.• • Inductively coupled plasma (ICP), also used to detect metals. Together these techniques• can be used to monitor all the metals of interest in wastewater.• • Gas Chromatography (GC), which is used to detect certain organic species.• Specific tests are used to monitor a wide variety of substances, conglomerations of

substances• and more general properties of the wastewater.

• • Elements (phosphorous and nitrogen)• • Molecules (ammonia and dissolved oxygen)• • Ions (nitrite, nitrate and sulphide)• In addition, some conglomerations of

substances (which include many individual chemical

• species) are monitored:• • Solids content (both suspended and total)

• And finally, some parameters of the solution as a whole are tested for:• • pH• • Alkalinity• • BOD5 (the amount of oxygen consumed by biological degredation processes over a

five• day period)• • COD (the amount of oxygen required to completely oxidise the chemical species

present)• • Turbidity• There are different test frequencies for different sample sites. Tests such as BOD and

suspended• solids are usually carried out frequently, e.g. daily or weekly, while other tests such as

those for• metals are done only occasionally, e.g. monthly or quarterly. The tests are discussed

individually• below.

• ENVIRONMENTAL IMPLICATIONS• Odour control• In the absence of adequate oxygen bacteria in the wastewater break

down essentially odourfree• compounds to odorous compounds: fats and carbohydrates go to

alcohols, esters,• aldehydes and carboxylic acids while proteins go to ammonia, amides,

mercaptans and• hydrogen sulphide. All of these compounds can give off strong smells,

but those formed• from protein degredation can emit very intense smells at

concentrations in the parts per• billion range.

• The foul air containing these compounds is mostly formed in the pretreatment and primary

• treatment phases. For this reason, the equipment used in these phases, as well as the fixed

• growth reactors and the sludge dewatering plant, are roofed over and the gases produced in

• these areas are removed using extractor fans. They are piped to one of six 'earth filters':

• mounds of soil and finely ground sand and scoria with a total surface area of 9800 m2 and an

• average total depth of 1m. The air is released from perforated pipes underneath the soil and

• percolates upwards. Organisms convert sulfurous compounds to sulfur, nitrogenous ones to

• nitrogen and organics to CO2 and CH4.• • Oil and grease (which includes anything that can dissolve in the solvent used

Figure 2–5: Primary settling tank schematic

• Sludge digestors2.15• 2.15The sludge which settles in the sedimentation basin is

pumped to the sludge digestors (see figure 2–7) where a temperature of 30–35ºC is maintained. This is the optimum temperature for the anaerobic bacteria (bacteria that live in an environment that does not contain oxygen). The usual length of digestion is 20–30 days but can be much longer during winter months. Continual adding of raw sludge is necessary and only well-digested sludge should be withdrawn, leaving some ripe sludge in the digestor to acclimatise the incoming raw sludge.

Figure 2–7: Sludge digestors

• Drying beds2.16• 2.16Digested sludge is placed on drying beds

of sand (see figure 2–8) where the liquid may evaporate or drain into the soil. The dried sludge is a porous humus-like cake which can be used as a fertiliser base.

• Trickling filters2.17• 2.17The liquid effluent from the primary settling tank is passed to the secondary part

of the system where aerobic decomposition completes the stabilisation. For this purpose, a trickling filter (see figures 2–9 and 2–10) is used.

• 2.18A trickling filter is a fixed bed, biological filter that operates under (mostly) aerobic conditions. Pre-settled wastewater is ‘trickled’ or sprayed over the filter. As the water migrates through the pores of the filter, organics are degraded by the biomass covering the filter material.

• 2.19The Trickling Filter is filled with a high specific surface-area material such as rocks, gravel, shredded PVC bottles, or special pre-formed filter-material. A material with a specific surface area between 30 and 900m2/m3 is desirable. The filter is usually 1–3 m deep but filters packed with lighter plastic filling can be up to 12 m deep. Pre-treatment is essential to prevent clogging and to ensure efficient treatment. The pre-treated wastewater is ‘trickled’ over the surface of the filter. Organisms that grow in a thin bio-film over the surface of the media oxidize the organic load in the wastewater to carbon dioxide and water while generating new biomass.

• Figure 2–9: Trickling filter • The incoming wastewater is sprayed over the

filter with the use of a rotating sprinkler. In this way, the filter media goes through cycles of being dosed and exposed to air. However, oxygen is depleted within the biomass and the inner layers may be anoxic or anaerobic.

• circulate. Whenever it is available, crushed rock or gravel is the cheapest option. The particles should be uniform such that 95 per cent of the particles have a diameter between seven and 10 cm. Both ends of the filter are ventilated to allow oxygen to travel the length of the filter. A perforated slab that allows the effluent and excess sludge to be collected supports the bottom of the filter.

• The bed consists of crushed rock or slag (1–2 m deep) through which the sewage is allowed to percolate. The stones become coated with a zoogloea film (a jelly-like growth of bacteria, fungi, algae, and protozoa), and air circulates by convection currents through the bed. Most of the biological action takes place in the upper 0.5 m of the bed. Depending on the rate of flow and other factors, the slime will slough off the rocks at periodic intervals or continuously, whenever it becomes too thick to be retained on the stones. A secondary settling basin is necessary to clarify the effluent from the trickling filter. The overall reduction of BOD for a complete trickling filter system averages around 80–90 per cent.

• Secondary settling tank2.23• 2.23With the majority of the suspended material removed from

the sewage, the liquid portion flows over a weir at the surface of the secondary settling tank (see figure 2–11). Chlorination of the effluent from the secondary settling tank takes place in accordance with state and local laws. Depending on the location most laws require that a free available chlorine (FAC) residual (usually 0.2 mg/L) be maintained after a 30-minute contact period. This contact period is obtained through the use of chlorine contact chambers which are designed to provide a 30-minute detention time. From the chlorine contact chamber the treated sewage is normally discharged into a receiving body of water.

• Activated sludge system2.24• 2.24Activated Sludge is a multi-chamber reactor unit that makes use of (mostly)

aerobic microorganisms to degrade organics in wastewater and to produce a high-quality effluent. To maintain aerobic conditions and to the keep the active biomass suspended, a constant and well-timed supply of oxygen is required. Activated sludge systems (refer figures 2–12 and 2–13) normally make use of bar screens and/or comminutors, grit chambers, primary settling tanks, secondary settling tanks, and digesters, which are operated in the same manner as those of trickling filter systems. They differ from the trickling filter systems in that they make use of an aeration tank instead of a trickling filter.

• 2.25Different configurations of the Activated Sludge process can be employed to ensure that the wastewater is mixed and aerated (with either air or pure oxygen) in an aeration tank. The microorganisms oxidize the organic carbon in the wastewater to produce new cells, carbon dioxide and water. Although aerobic bacteria are the most common organisms, aerobic, anaerobic, and/or nitrifying bacteria along with higher organisms can be present. The exact composition depends on the reactor

• design, environment, and wastewater characteristics. During aeration and mixing, the bacteria form small clusters, or flocs. When the aeration stops, the mixture is transferred to a secondary clarifier where the flocs are allowed to settle out and the effluent moves on for further treatment or discharge. The sludge is then recycled back to the aeration tank, where the process is repeated.

• 2.26Compressed air is continually diffused into the sewage as it flows through the aeration tank. This provides both a source of oxygen for the aerobic bacterial floc that forms in the tank and the turbulence necessary to bring the waste and the bacteria into contact. Aerobic bacteria attack the dissolved and finely divided suspended solids not removed by primary sedimentation. Some of the floc is removed with the sewage that flows out of the aeration tank and carried into the secondary settling tank. Here the floc settles to the bottom of the tank, and is later pumped back into the aeration tank. The liquid portion then flows over a weir at the surface of the settling tank to be chlorinated and released to a receiving stream.

• 2.27To achieve specific effluent goals for BOD, nitrogen and phosphorus, different adaptations and modifications have been made to the basic Activated Sludge design. Aerobic conditions, nutrient-specific organisms (especially for phosphorus), recycle design and carbon dosing, among others, have successfully allowed Activated Sludge processes to achieve high treatment efficiencies.

• Rotating biological contactor system2.28• 2.28Rotating biological contactor systems (refer figures 2–13, 2–14 and 2–15)

normally make use of bar screens and/or comminutors, grit chambers, primary settling tanks, secondary tanks, and digesters, which are operated in the same manner as those of trickling filter systems. The rotating biological contactor (RBC) is a simple, effective method of providing secondary wastewater treatment. The system consists of biomass media, usually plastic, that is partially immersed in the wastewater. As it slowly rotates, it lifts a film of wastewater into the air. The wastewater trickles down across the media and absorbs oxygen from the air. A living biomass of bacteria, protozoa, and other simple organisms attaches and grows on the biomass media. The organisms then remove both dissolved oxygen and organic material from the trickling film of wastewater. Any excess biomass is sloughed-off as the media is rotated through the wastewater. This prevents clogging of the media surface and maintains a constant microorganism population. The sloughed-off material is removed from the clear water by conventional clarification. The RBC rotates at a speed of one to two rpm and provides a high degree of organic removal.