Municipal Wastewater Treatment Methods : Primary, Secondary, Advanced and Final treatment

 Municipal wastewater treatment is basically applicable to those wastewater which has been collected from various residential localities through the sewage system of a city and town, and is treated before being dispersed in open field.
  Municipal wastewater treatment plants carryout a series of treatment processes which may be summarized as follows:

i. Primary treatment :

  As the name indicates, it is a primary treatment or physical treatment. This treatment is carried out to remove coarse solids and settleable solids.

ii. Secondary treatment :

  It is known as biological treatment. It comes after primary treatment. The aim of secondary treatment is to oxidize organic constituents of the wastewater by means of microorganisms and reduction of BOD also takes place.

A Typical Sewage Treatment Plant
A Typical Sewage Treatment Plant

iii. Advanced treatment :

This treatment is carried out for the removal of additional objectionable substances to further reduce BOD. The treatment Includes removal of nutrients such as phosphorus and nitrogen that are responsible for algal blooms.

iv. Final treatment :

   It is also known as Chemical treatment. This is basically to disinfect and dispose of liquid effluents.
  Solid processing involves the treatment of sludge or biosolids for stabilization of its organic matter and water content. It can then be used as organic manure for agriculture.

(A) Primary Treatment and Secondary Treatment : Principles and Role of Microorganisms

i. Primary Treatment :

  This is also known as physical treatment. It consists of methods to remove solid materials and these methods are basically mechanical.

Wastewater as it arrives at a wastewater disposal plant is first treated to remove coarse solid materials by a variety of mechanical techniques which include-
(a) Screening
(b) Grinding
(c) Floatation
(d) Sedimentation

(a) Screening :

In this method, the wastewater collected from various residential areas is passed through various sizes of screens such as bar rock, fine screens and microscreens.

The solid objects which are larger than the screens will be retained back.

(b) Grinding :

Here, the solid mass is ground or broken down into smaller particles, which will settle down in grit chambers.

(c) Floatation :

Certain substances which are light in weight and floats on the surface such as finely divided suspended solids, particles with densities close to that of water, oil and gases are removed by dissolved or induced air floatation.

(d) Sedimentation :

Sedimentation units such as tanks, basins or mechanical device provide the means for concentrating and collecting the particulate material referred to as sludge.

Following sedimentation, the sludge and the liquid effluent are processed separately.

Secondary treatment does not taken place before removal of coarse and settleable particles. If they are not removed, then they cause interference in later stages of secondary treatment.

ii. Secondary Treatment :  

  Secondary treatment processes accomplish oxidation of the organic material in the liquid waste by microbial activity.

During this process microorganisms transform organic compounds into simple compounds.

For the complete breakdown of organic compounds, oxygen supply is necessary. If there is no supply of oxygen, then the process is anaerobic and the degradation is incomplete. For the efficient treatment, sufficient oxygen supply is required.

Many different methods are designed for secondary treatment in such a way that  maximum amount of sewage is exposed to air because wastewater can hold a limited amount of oxygen.

Oxygen is not provided by artificial way because it is costly.

The ultimate aim of secondary treatment is to reduce BOD in the wastewater. About 80 to 90% of the BOD and pathogens are removed.

There are various methods for secondary treatment.

(a) Septic Tank :

  A septic tank is a sewage - settling tank designed to retain the solids of the sewage entering the tank long enough to permit adequate decomposition of the sludge.

Septic Tank
Septic Tank

Thus, the unit accomplishes two processes.

(1) Sedimentation, and
(2) Biological degradation of the sludge.

As sewage enters this type of tank, sedimentation occurs from the upper portion, permitting a liquid with fewer suspended solids to be discharged from the tank.

The sedimented solids are subjected to degradation by anaerobic bacteria, hence the end products are still unstable i.e. high in BOD and odorous.

When microbial growth in completed, it forms a stable settleable structure called flocs.

The effluent from the septic tank is distributed under the soil surface through a disposal field as shown in the figure.

Septic tank is the most satisfactory method for disposing of sewage from small installations, especially individual dwellings and isolated rural buildings where public sewen are not available.

They cannot however, be relied upon to eliminate pathogenic microorganisms carried in the sewage.

Consequently, it is imperative that the drainage from the tank be prevented from contaminating the drinking water supply.

(b) Imhoff Tank :

The Imhoff tank was developed to correct the two main defects of the septic tank.

Imhoff Tank
Imhoff Tank
- It prevents the solids once removed from the sewage from being mixed with it again, but still provides for the decomposition of these solids in the same unit.
- It provides an effluent amenable to further treatment.

The Imhoff tank may be either circular or rectangular and is divided into three compartments: 

(1) The upper section or sedimentation compartment.
(2) The lower section known as the digestion compartment.
(3) The gas vents and scum section.

It is desirable to be able to reverse the direction of flow to prevent excessive deposition of solids at one end of the sediment compartment.

Periodically reversing the flow will result in an even accumulation of sludge across the bottom of the tank.

In operation, all of the wastewater flows through the upper compartment.

Solids settle to the bottom of this sloped compartment slide down and pass through an opening or slot to the digestion compartment.

One of the bottom slopes extends at least 6 inches beyond the slot. This forms a trap to prevent gas or digesting sludge particles in the lower section from entering the waste stream in the upper section.

The gas and any rising sludge particles are diverted to gas vents and scum section.

(c) Trickling Filter :

  The trickling filter consists of a bed of crushed stone, gravel, slage or synthetic material with drains at the bottom of the tank.

Trickling Filter
Trickling Filter 

Trickling filters have a depth of 5 to 10 feet. The circular round of the trickling filter is filled with various sizes of stones.

In the centre of trickling filter, there is an arm running on entire bed.

The liquid sewage is sprayed over the surface of the bed either by a rotating arm or through nozzles.

The spraying saturates the liquid with oxygen. The treatment in trickling filter is an aerobic process.

Wastewater is kept on the bed of stones, stones are of different shapes so the air pockets between the arrangement of stones varies.

The sewage trickles from these air pockets. Now the rotating arm is rotated and begins a new entire cycle. The time is noted for the sewage to pass through the bed.

Sewage contains bacteria, that adheres on the stones. The sewage which falls on stones is rich in nutrition, organic compounds and air is sufficient in this system.

Now the air pockets will totally be aerobic and microorganisms will grow, consume the oxygen as well as nutrients. Then microorganisms multiply slowly. This results in formation of film of microorganisms called zoogloeal film.

Due to this layer the rate of filtration decreases. This layer is made up of millions of microorganisms, so Initially the rate of filtration is fast, but with time filtration rate decreases.

The sewage falling from rotating arms is rich in organic compounds and nutrition, which gives nutrient for growth of microorganisms. The water collected from the bottom has about 80% reduction of BOD. This is a very good process for the treatment of wastewater.

The slimy layer formed is composed of a mixed microbial community including bacteria such as Beggiatoa alba, Sphaerotilus natans, Achromobacter sp., Pseudomonas sp., Flavobacterium sp., and Zoogiea ramitera, etc., Fungi, protozoa, nematodes and rotiflers are also present .

(d) Activated Sludge Process :

  The activated sludge process is an aerobic suspension type of liquid waste treatment system, also known as aeration tank digestion.

Activated Sludge Process
Activated Sludge Process

After primary treatment, the sewage containing dissolved organic compounds is introduced into an aeration tank and mixed with a slurry rich in bacteria which is called an activated sludge.

Air injection and or mechanical stiring provides the aeration.

The heterogenous nature of the organic substrates in the sewage allows the development of diverse heterotrophic bacterial populations Including gram-negative rods, predominantly Escherichia, Enterobacter, Pseudomonas, Achromobacter, Flavobacterium and Zooglea sp., other bacteria includes Micrococcus, Arthrobacter, Sphaerotilus and other large filamentous bacteria, and low numbers of filamentous fungi, yeasts and protozoa, mainly ciliates.

The protozoa are important predators of the bacteria alongwith rotifers.

The bacteria in the activated sludge tank occur in free suspension and as aggregates or flocs.

The flocs are composed of microbial biomass held together by bacterial slimes produced by Zooglea ramigera and similar organisms.

The floc is too large to be ingested by the ciliates and rotifers.

In the raw sewage, suspended bacteria predominate, but during holding time in the aeration tank, their number decreases and at the same time those bacteria associated with flocs greatly increase in number.

As a result, significant portion of the dissolved organic substrate is mineralized and another portion is converted to microbial biomass.

In the advanced stage of aeration, most of the microbial biomass becomes associated with flocs that can be removed from suspension by settling in a settling tank.

Sometimes because of lower O2 levels or due to too old or young microbial population proliferation of filamentous bacteria like Sphaerotilus, Beggiatoa, Thiothrix and other filamentous fungi causes bulking of sewage, which results in formation of flocs that do not settle this affects the effluent quality.

A portion of the settled sewage sludge is recycled for use as the inoculum for the incoming raw sewage.

The organisms present in the flocs reduces the time for treatment everytime it is recycled.

During the treatment, BOD level is reduced by 85-90%.

(e) Oxidation Ponds :

  Oxidation ponds are also called Lagoons or stabilization ponds.

Oxidation ponds
Oxidation pond 

This pond is natural or artificial. It is basically shallow pond and having a large surface area and depth of 2 to 4 feet.

In an oxidation pond, wastes are added at single point, either in the middle of the pond or at the edge of the pond and effluent is removed at the single point at the edge of the pond.

An oxidation pond is an aerobic system for the simple secondary treatment of the waste water in small industrial units or villages. However, the pond is having both aerobic as well as anaerobic zones.

Within an oxidation pond, heterotrophic bacteria degrade organic matter in the sewage, which results in production of cellular material, CO2 and minerals.

The production of these substances supports the growth of algae in the oxidation ponds.

The production of this oxygen by photosynthetic activity of algae replenishes the oxygen used by the heterotrophic bacteria.

The performance of oxidation pond is strongly influenced by seasonal temperature.

Oxidation ponds also tend to get filled due to the settling of bacteria and algal cells formed during the decomposition of sewage.

Overall, oxidation ponds, are low-cost operations but they tend to be inefficient and requires large holding capacity and long retention time.

(B) Advanced Treatment, Final or Tertiary Treatment and Efficiency of Waste Treatment Procedures :

i. Advanced Treatment :

  The advanced treatment is needed when the organic matter in sewage which has passed through primary and secondary treatment is not yet completely degraded.

Various kinds of advanced treatments are there which depends on the type of incoming sewage.

Unit processes have been developed to remove nutrients, simple organic substances and complex synthetic organic compounds.

Processes Include biological as well as physico-chemical methods.

Examples are, Biological nitrification-denitrification, filtration, reverse osmosis, carbon adsorption, chemical addition and lon-exchange, etc.

The major disadvantage of advanced treatment processes Is its high cost.

(a) Biodisc system or rotating biological contactor

This process is an example of a more advanced type of aerobic film-flow treatment system. In this process closed packed plastic discs, partially submerged in a trough containing the sewage are rotated.

Microbial slime forms on the surface, which becomes thick and finally sloughs off when the Inner most microbes die and the film detaches from the plastic discs.

It is useful In treatment of domestic as well as industrial wastewater.

(b) Modified or non-conventional activated process

This process is employed for removal of nitrogen and phosphorus, which are generally removed during tertiary treatment.

A single sludge system comprising of a series of aerobic and anaerobic tanks is one method, In which methanol or settled sewage is used as the carbon source by denitrifiers.

In multisludge system three separate systems are involved in carbonaceous oxidation, nitrification and denitrification.

Bardenpho process removes nitrogen as well as phosphrus during nitrification-denitrification process. It consists of two aerobic and two anoxic tanks followed by a sludge settling tank.

ii. Final or Tertiary Treatment and Efficiency of Waste Treatment Procedures:

  After final treatment, the water is to be released in water bodies which is to be used by living creatures.

Disinfection is commonly accomplished by chlorination. However, current research has proved the serious impact chlorinated waters have on the aquatic life of the receiving water.

This has led to the development of several disinfection alternatives. The use of ozone and ultra-violet light is becoming more prevalent.

Many facilities that continue to employ chlorine for disinfection now include dechlorination prior to discharge into a water body.

Dissolved oxygen may also be added to the treated wastewater prior to final discharge. This process is termed post aeration that is accomplished by mechanical means or a cascading slow techniques, which minimizes the decrease in dissolved oxygen of the receiving water bodies.

Activated carbon filters are normally used for the removal of P and N from secondary treated effluents.

Phosphate is removed by precipitation as calcium, aluminium or Iron phosphate. Its presence in effluents causes eutrophication of receiving water bodies. It can be achieved by addition of lime.

Ammonia (NH3) is removed by Break-point chlorination process, in which it is converted to form dichloramine and then to molecular nitrogen.

Nitrogen can also be removed by denitrification. Here during aerobic treatment nitrate is produced by nitrifying bacteria. Nitrate is then reduced to nitrogen gas and nitrous oxide (N2O) by denitrifying bacteria that use it as an electron acceptor during degradation of organic matter under low oxygen condition.

Anammox process is an anaerobic nitrogen removal process in which ammonium ion (used as the electron donor) is allowed to react with nitrite (the electron acceptor) produced due to partial nitrification. It can convert about 80% of the ammonium ion initially present to nitrogen gas.

Tertiary treatment processes are expensive and hence used only where it is justified.

Removal of pathogens :

Pathogens present in raw sewage are successfully removed mainly during the activated sludge process.
It removes viruses, enteric bacterial pathogens (e.g. Salmonella), Glardia and Cryptosporidium.
Chlorination, ozonization and U.V. rays are employed during tertiary treatment process for killing pathogens.

(C) Solid Waste Processing : Anaerobic Sludge Digestion and Composting

  A major cost at modern large scale wastewater treatment is associated with additional processing. of effluent after tertiary treatment.
Solid waste or sludge processing, removal of pathogens, composting, land-farming, etc. are involved in this activity.

i. Anaerobic Sludge Digestion :

  The biosolids or sludge is produced during the sewage treatment stages is rich in organic matter.

The primary sludge contains about 3.8% solids, whereas secondary sludge contains 0.5 to 2.0% solids.

The steps involved in treatment of sludge includes:
Thickening : reduction in its volume, by settling in a tank or by centrifugation.
  Digestion : this is for stabilization of the organic matter by microbial activities. It also causes destruction of pathogens due to higher temperatures reached during the process.

It is carried out by both anaerobic or aerobic digestion.

The sludge which accumulates during various treatment stages is collected and pumped into a separate tank designed for its digestion under controlled conditions.

Initially, for a few hours, air is supplied for aerobic degradation. Aerobic microorganisms degrade the sludge reducing the solids content, and create anaerobic condition.

Anaerobic digestion takes place for a period of 2-3 weeks in a large covered tank, it results in the formation of gases namely, methane (60-70%), CO2 (20-30%), small amounts of hydrogen and nitrogen. 

Methane produced is used as a biofuel (biogas).

This causes further stabilization of the sludge.

Ripe sludge which contains an actively growing population of diverse microorganisms is used to seed fresh sludge collected in tanks. This helps is shortening the time Ens noj required for establishment of microbes and digestion.

Conditions such as pH, temperature, nutrients, etc. which affect microbial metabolism and growth have to be controlled to achieve proper digestion.

The process of adding chemicals such as lime, alum, ferric chloride, etc. for reduction of solids, precipitation of phosphorus, etc. is employed.

 Finally, air drying, centrifugation or vacuum filtration are employed for dewatering the solids, to decrease its volume and reduce transportation cost.

Land farming is a practice for disposal of biosolids produced during wastewater treatment onto agricultural land.

Processes for reduction of pathogens are used to treat sludge before adding to soil.

The blosolids may be liquid or solid in nature for soll application. It adds water and nutrients to soil that promotes soil fertility and plant growth.

Various types of sanitary landfill, are employed for disposal of municipal solid waste (MSW) generated in major cities. Both organic and Inorganic solid wastes are deposited in a low-laying waste-land, and covered with a layer of soil over everyday's waste added.

Finally, a developed landfill may be used for recreation or construction purpose.

ii. Composting

Composting is a process for biodegradation or decomposing organic solid waste by aerobic, mesophilic and thermophilic microorganisms.

Dewatered sludge or solid organic wastes after removal of inorganic fraction is ground, mixed with sludge or bulking agents such as wood chips, shredded newspaper, resins, etc. and composted.

Microbial degradation activities converts the waste Into a stable humus like product.

In Window method the solid waste is piled up in long rows, covered and allowed to decompose. The material is turned over at regular intervals.

The Static pile or Aerated pile method waste is piled up on perforated pipes through which air is pumped. This speeds up the composting process.

At some modern facilities, an enclosed reactor composting under controlled conditions is carried out in an industrial bioreactor or fermenter. The composting occurs in 2-4 days, however, it is expensive.

For optimal composting the C: N ratio must be 40:1, temperature 50 - 60°C, and moisture 50-60%.

Mesophilic bacteria and fungi grow first followed by thermophilic organisms such as Thermomonospora spp., Clostridium spp., Aspergillus fumigatus, Geotrichum spp., Torula thermophila, etc.

The high temperature kills most of the enteric pathogens present during composting of solid waste.

The composted waste is screened for removal and recycling of wood chips, resins, etc. and humus like material is used as a soil conditioner as a commercial product, especially for organic farming.