Sewage: Meaning, Composition and Disposal

In this article we will discuss about:- 1. Meaning of Sewage 2. Composition of Sewage 3. Classification 4. Characteristics 5. Disposal 6. Treatment.

Meaning of Sewage:

‘Sewage’ is a collective noun used to represent liquid or solid wastes carried in sewers. It consists of domestic water-borne wastes including human and animal excrete, washing waters and everything that goes down the drains of a town or a city. It also consists of industrial water-borne wastes as well as ground, surface and atmospheric waters which enter the sewerage system.

The amount of sewage produced in our country is of the order of 3.61 million cubic metres/day (about 800 million gallons/day). About 30% of the above amount comes from urban areas. It is estimated that only about 20% of one day sewage production of our country is treated and utilized, and the rest (about 80%) still remains untreated and unutilized.

Composition of Sewage:

The composition of sewage mainly depends upon per capita consumption of water and varies from place to place and season to season.

The sewage composition can be studied under following two heads:

1. Chemical Composition:

Chemically, the sewage consists of approximately 99% water and 1% inorganic and organic matter in suspended and soluble forms. Lignocellulose, cellulose, proteins, fats, and various inorganic particulate matter exist in suspended state, whereas sugars, fatty acids, alcohols, amino acids, and inorganic ions constitute the soluble forms.

However, on an average, the sewage of towns in our country contains about 350 ppm biodegradable organic matter, 52 ppm N₂, 45 ppm potassium and 16 ppm phosphorus. Salts of several heavy metals such as Zn, Cr, Ni, Pb, etc. are also present above permissible levels in sewage.

2. Microbial Composition:

The microbial population per millilitre of sewage may vary from a few lacs to several millions. Various types of microorganisms, viz., micro-fungi, bacteria and protozoa, collectively called ‘sewage fungus’, are known to grow profusely in sewage.

In addition, viruses and many micro-algal genera have also been recorded from sewage. Bacteria occurring in sewage are mainly intestinal and soil inhabiting and their common types are coliforms, streptococci, Clostridia, micrococci, Proteus, Pseudomonas, and lactobacilli.

Classification of Sewage:

Sewage may be classified mainly into two types, namely, domestic and industrial. All household wastes and human and animal excrete constitute domestic sewage, whereas the industrial wastes constitute industrial sewage.

Since industrial wastes vary greatly in their composition (some may be highly alkaline such as soda wastes, some highly acidic such as acid-mine drainage, and others toxic because of presence of heavy metals, antibiotics, pesticides, etc.), the treatment of industrial sewage proves highly difficult in comparison to domestic sewage.

Characteristics of Sewage:

(i) Biochemical Oxygen Demand (BOD) and Oxygen Consumption (OC) values are extremely high in sewage.

(ii) The sewage organic matter undergoes anaerobic or partial decomposition resulting in the production of obnoxious gases, namely, CH₃, CO and H₂S due to anoxic condition. Besides being toxic, these gases react with water and produce acids.

(iii) Production of acids in large quantity make the sewage more acidic thus making it unfit for supporting life activities.

(iv) Heavy metals are generally present in abnormal concentration in sewage.

All these characteristics of sewage, viz., anoxic condition, high acidity, high heavy metal concentration, and reduced photosynthetic rate due to poor illumination cause death of oxygen-dependent organisms such as aerobic microorganisms, plants and animals in sewage. This is the reason why sewage is dominated by organisms capable of growing in anaerobic environments.

Disposal of Sewage:

Sewage disposal has become of prime importance now-a-days as it brings undesirable and harmful effects on living beings. Untreated or inadequately treated sewage is generally disposed of into natural water reservoirs without taking its pros and cons into account.

It is so either because we are indifferent to the consequences or because we assume that the water reservoirs are sufficiently large and so located that sewage-dilution prevents hazards.

However, we can no longer rely on disposed-sewage dilution in our natural water reservoirs; the solution of sewage pollution is not its dilution. It is necessary, therefore, that the sewage must be treated before its disposal so that we can, on one hand, save organisms including men from bad effects and, on the other hand, can utilized it to the maximum for our welfare.

Disposal of sewage as such or inadequately treated one, generally leads to following consequences:

(i) Frequent dissemination of water-borne disease causing microorganisms in large number.

(ii) Depiction of dissolved oxygen in water leading to anoxic (oxygen-less) condition which may ultimately kill O₂ dependent aquatic life.

(iii) Creation of offensive odour and debris-accumulation due to which value of property decreases.

(iv) Increased danger of swimming in water and diminished value of water for other recreational purposes.

Treatment of Sewage:

Our objectives behind the sewage treatment would be to kill pathogenic microorganisms, prevent anoxia, raise the pH to alkaline side, increase photosynthetic rate, reduce organic content, etc. When these objectives are achieved by the way of treating the sewage, the conditions prevailing in a natural water reservoir are induced in sewage water and the latter can be reused.

Sewage treatment processes are many and varied. We will discuss only those sewage treatment processes which are generally applied in single dwelling unit situations and municipal situations.

Single Dwelling Unit Treatment Processes:

1. Outdoor Toilets:

Where plumbing installations cannot be undertaken for any reason, the toilets or water closets may be constructed outdoors. While this arrangement is undertaken, care could be taken to see that flies have no access to these and changes of drainage from these, joining water supplies, are eliminated.

2. Septic Tanks:

These are used for residential quarters (Fig. 32.1). All the residential sewage is passed through suitable pipes leading to a tank located at a suitable place and made of metal or concrete. The heavy particles of sewage settle down and undergo anaerobic decomposition whereas the gases and clear water are allowed to go out through perforated pipes ramified within the ground.

The septic tank device should be so fitted that the sewage does not drain by any chance into water supply of the residence. The sludge in the tanks must be periodically removed to prevent clogging of the pipes.

3. Imhoff Tank:

This is, in fact, a modification of septic tank and is generally used to treat larger community sewage (Fig. 32.2). It consists of two chambers, one above the other. The top chamber receives sewage and the heavier particles settle into the lower chamber and slowly decompose under anaerobic conditions.

The gas liberated (mainly methane) can be drawn out through a passage and utilized as fuel. The sewage effluent (remaining sewage water) is either let into larger body of water, or is subjected to aerobic decomposition. The sludge is periodically removed, aerated and used as manure.

Municipal Treatment Processes:

Municipal sewage treatment systems carry out various steps involved (Fig. 32.3). These steps are, namely, primary (or mechanical) treatment, secondary (or biological) treatment, and tertiary (or final) treatment.

1. Primary (or Mechanical) Treatment:

When the sewage arrives at a sewage treatment plant, it is first subjected to mechanical (or physical) means, viz., flowing, dilution and sedimentation to remove its coarse solid materials. The sewage is passed through a series of filters of graded openings and then allowed to flow through sedimentation units (tanks, basins, etc.).

Coarse solid materials are concentrated in and collected from sedimentation units; these particulate materials are collectively called ‘sludge’. Following sedimentation, the sludge and liquid affluent are processed separately during secondary treatment.

2. Secondary (or Biological) Treatment:

This is purely a biological treatment of mechanically treated sewage and concerns microbial activity which biodegrades organic substrates and oxidizable inorganic compounds. This treatment accomplishes two important phases, namely, aerobic phase and anaerobic phase.

The aerobic phase consists of aerobic digestion of sludge by various filters (e.g., trickling filters), oxidation ponds and activated sludge process, and the anaerobic phase is represented by anaerobic digestion of sludge.

I. Aerobic Phase of Secondary Treatment:

(i) Aerobic Digestion in Trickling Filters:

Trickling filter consists of generally 6-10 feet deep bed of crushed stone, gravel, slag, or similar material. The sewage effluent is sprayed over the surface of the bed; the spraying saturates the effluent with oxygen. The bed surface becomes coated with aerobic microbial flora consisting of microalgae, micro-fungi, bacteria, and protozoa.

As the effluent seeps over, the aerobic microbes degrade the organic matter. However, the treated effluent collected at the bottom of the tank is passed to sedimentation tank and, like activated sludge process, the effluent follows tertiary treatment. Aerobic digestion of sewage organic matter in a trickling filter is a very slow process.

(ii) Oxidation Ponds:

Oxidation pond sewage-treatment is recommended for small communities in rural areas where suitable and sufficient land is available. Oxidation ponds (also called Lagoons or Stabilization Ponds) are generally 2-5 feet deep shallow ponds designated to allow direct wind action and algal growth on the sewage effluent.

Oxygen supplied from air and produced as a result of algal photosynthesis fulfils biochemical oxygen demand (BOD) of sewage effluent and thus helps in maintaining aerobic condition in sewage effluent. In such condition the aerobic microbes grow rapidly and digest organic matter. Chlorella pyrenoidosa is a common algal representative grown in oxidation ponds.

(iii) Activated Sludge Process:

In this process, the mechanically treated sewage effluent (serge liquid) is pumped into a sedimentation or settling tank wherein the sewage floes and settles out. A portion of sewage ‘floe’ is returned to activate a new batch of mechanically treated sewage effluent, and the rest is pumped to activated sludge digester where air is blown by several jets.

Thus, in the presence of plentiful oxygen, oxidation of sewage effluent is brought about by aerobic microorganisms which break down organic matter to CO₂ and H₂O. Now the effluent is passed through a sedimentation tank. Though about 90% of the organic matter of the effluent is digested via this process, the effluent still contains considerable amount of nitrate and phosphate, etc.

It is, therefore, not safe to discharge effluent at this stage into a large body of water ds both nitrate and phosphate can cause eutrophication. Now the effluent, which looks clear at this stage, is subjected to tertiary (final) treatment for further purification.

II. Anaerobic Phase of Secondary Treatment (Anaerobic Digestion of Sludge):

The sludge collected after primary (mechanical) treatment of sewage is subjected to anaerobic (oxygen- free) digestion in separate tank designed especially for the purpose. Since anaerobic condition prevails in this tank, the anaerobic microbes bring about digestion of organic matter by degrading them to soluble substances and gaseous products (methane, 60-70%; CO₂, 20-30%; and smaller amounts of H₂ and N₂).

This gas mixture can be used for operating power for the sewage plant or as a fuel. Recently, Municipal Corporation of Delhi has started supplying this gas mixture to about 100,000 people for cooking purposes.

3. Tertiary (or Final) Treatment:

Since the sewage-effluent treated during secondary treatment process still contains non-biodegradable organic pollutants (if sewage contains industrial wastes) and mineral nutrients particularly nitrogen and phosphorus salts, it is subjected to tertiary (or final) treatment for their removal.

If not so, the sewage effluents containing nitrogen and phosphorus salts can cause serious eutrophication in aquatic ecosystems. Non-biodegradable organic pollutants are normally removed by using activated carbon filters whereas phosphorus and nitrogen salts by chemical treated.

Phosphorus salts are precipitated by liming and the nitrogen present mainly as ammonia is removed by volatilization (vigorous aeration at elevated temperature) at a high pH. These treatments result in a high-quality effluent which does not cause eutrophication.

The find step of tertiary treatment is disinfection which is commonly accomplished by chlorination using either sodium or calcium hypochlorite (NaOCl or CaOCl₂ respectively) or chlorine. Now the effluent is a clean water and is considered microbiologically safe even for human consumption.