Chemical-Aided Sedimentation of Sewage | Waste Management

Chemical-aided sedimentation of sewage is analogous to chemical coagulation, flocculation and sedimentation in water treatment. The colloidal and finely dispersed solids which cannot be removed by plain primary sedimentation alone, as they possess extremely low settling velocities, are aggregated into settleable particles by addition of chemicals known as coagulants in chemical-aided sedimentation.

Chemical-aided sedimentation involves the unit processes and operations of chemical coagulation, flocculation and sedimentation. The colloidal and finely dispersed solids present in sewage are usually negatively charged and therefore do not agglomerate naturally into settleable mass.

Addition of chemical coagulants results in destabilization, aggregation and binding together of colloidal and finely dispersed solids by any one or more than one mechanisms of-

(i) Ionic Layer compression;

(ii) Adsorption and charge Neutralization;

(iii) Enmeshment in Precipitate; and

(iv) Interparticle Bridging.

When chemical coagulant is added to sewage and thoroughly mixed with it an insoluble, gelatinous, flocculent precipitate called floe is formed. The floes are the polymers of hydroxometal complexes which are heavy and settleable.

The floes are adsorbed on colloidal and finely dispersed solids and neutralize their charge or form bridging between colloidal and finely dispersed solids if the polymers are of the same charge as that on colloidal and finely dispersed solids. Further colloidal and finely dispersed solids can get entrapped within the flocs.

Chemicals Used as Coagulants:

Certain trivalent and divalent metallic salts are the most commonly used chemical coagulants which include iron salts such as ferrous sulphate, ferric chloride, ferric sulphate and chlorinated copperas; aluminium salts such as aluminium sulphate (or alum) and aluminium chloride; and lime and sodium carbonate.

Iron Salts:

Ferric salts are better coagulants than ferrous salts because of their higher valency and their efficiency over a wider pH range. Ferric salts are effective at all pH values above 3, the efficiency increasing with increase in pH, while the useful pH range of ferrous salts is above 10.

But when sewage is highly alkaline due to presence of industrial wastes, it may be cheaper to use larger dosage of ferrous salts as they are relatively cheaper. Chlorinated copperas which is an equimolar mixture of ferric sulphate and ferric chloride formed by the addition of chlorine to ferrous sulphate is also used in place of ferric salts.

Aluminium Slats:

Aluminium sulphate (or alum) and aluminium chloride are the commonly used aluminium salts. Where alum is used, the sludge produced is greater in volume and also bulky than with iron salts making it less easily settleable.

Lime and Sodium Carbonate:

These are used for pH adjustment to favourable ranges of coagulants especially when sewage is highly acidic. Lime is sometimes used independently as coagulant, particularly when iron pickling liquors are present in sewage. The action may be due to formation of calcium carbonate floe or reactions with small amounts of aluminium or iron salts present in sewage. Lime incidently helps in grit separation, oil and grease removal and is perhaps the cheapest chemical used as coagulant in chemical- aided sedimentation of sewage.

Table 12.4 gives the chemical formulae and molecular weights of chemical coagulants most commonly used in chemical-aided sedimentation of sewage.

Chemical-aided sedimentation produces intermediate results between plain sedimentation and secondary biological treatment. With proper dosages of chemical coagulants in chemical-aided sedimentation it is possible to remove 80 to 90% of suspended solids, 70 to 80% of BOD, and 80 to 90% of the bacteria.

Comparable removal values for well-designed and well-operated primary sedimentation tanks without the addition of chemical coagulants are 50 to 70% of suspended solids, 25 to 40% of BOD, and 25 to 75% of bacteria. Because of the variable characteristics of sewages, the required dosages of chemical coagulants should be determined from bench- or pilot-scale tests. In general the required dosages of the various chemical coagulants vary in the ranges as indicated in Table 12.5 which also shows the effectiveness of the various chemical coagulants.

Unit Operations in Chemical-Aided Sedimentation:

The chemical-aided sedimentation involves three unit operations viz.

1. Coagulation

2. Flocculation

3. Sedimentation

1. Coagulation:

It is the process of adding chemical coagulant to sewage and mixing it thoroughly with sewage. The required dose of chemical coagulant is fed to sewage by means of proportioning and feeding devices, ahead of the mixing unit. Mixing is accomplished in a rapid or flash mixing unit provided with paddles or propellers, or by diffused air and having detention period of 0.5 to 3 minutes.

The paddles or propellers of mixing unit are mounted on a vertical shaft and driven by a constant speed motor through reduction gears. The size and speed of the propeller is so selected as to give a propeller capacity of twice the maximum flow through the tank. The shaft speed is generally of the order of 100-120 rpm and power requirement is about 0.1 kW/Mld.

2. Flocculation:

It is the process by which the floccules that are formed after flash mixing of sewage with chemical coagulants are made to coalesce into bigger sizes. This is accomplished in flocculators.

There are various types of flocculators out of which mechanical flocculators are most commonly used and the same are described below:

Mechanical flocculators consist of tanks provided with paddles for stirring of sewage, and hence these are also known as paddle flocculators.

Depending on the direction of flow of sewage in the tank the mechanical flocculators may be classified as:

(i) Longitudinal flow flocculators, and

(ii) Vertical flow flocculators

A longitudinal flow flocculator consists of a rectangular tank provided with paddles mounted on a horizontal shaft. A vertical flow flocculator consists of a circular tank provided with paddles mounted on a vertical shaft. The paddles are rotated by an electric motor at a speed of 2 to 3 r.p.m.

The sewage coming from a mixing unit enters the flocculator tank through an inlet provided at one end of the tank and leaves it through an outlet provided at the opposite end. In a longitudinal flow flocculator both inlet and outlet are provided near the top of the tank. However, in a vertical flow tank inlet is provided at the bottom of the tank and outlet is provided near the top of the tank.

The design criteria for these flocculators are as follows:

(i) Depth of tank: 3 to 4.5m

(ii) Detention period: 10 to 40 minutes; normally 30 minutes

(iii) Velocity of flow in the flocculation tank: 02 to 0.8 m/minute, normally 0.4 m/minute

(iv) Total area of paddle: 10 to 25% of the cross-sectional area of the tank

(v) Peripheral velocity of blades: 0.2 to 0.6 m/s, normally 0.3 to 0.4 m/s

(vi) Power consumption: 10.0 to 36.0 kW/Mld

(vii) Outlet flow velocity to settling tank: 0.15 to 0.25 m/s (to prevent settling or breaking of floc as sewage flows through outlet pipe or channel)

In case of domestic sewage and certain industrial sewages, mechanical flocculation without addition of chemical coagulants will induce self-flocculation of the finely divided suspended solids and hence increase the efficiency of sedimentation.

3. Sedimentation:

The flocculated sewage solids are settled out in a subsequent sedimentation tank. In the design of these sedimentation tanks usually detention period of 2 hours and surface overflow rate or surface loading rate of not more than 50 m³/d/m² for average flows is adopted. Table 12.6 gives the recommended surface overflow rates for various chemical suspensions to be used in the design of sedimentation tanks.

Chemical-aided sedimentation is particularly advantageous where there is a large seasonal variation in sewage flow or as an emergency measure to increase the capacity of an overloaded plain sedimentation tank. However, chemical-aided sedimentation will not remove dissolved solids.

On the contrary, addition of chemicals which are soluble may add to the total dissolved solids concentration of sewage. Moreover as compared to secondary biological treatment methods such as trickling filter or activated sludge, chemical- aided sedimentation is less efficient. The other disadvantages of chemical-aided sedimentation are the increased costs and the considerable increase in sludge volume.

As such chemical-aided sedimentation is used only where-

(i) Seasonal treatment is required as in a summer resort, or variations in strength and volume of sewage are high.

(ii) There are seasonal variations in the concentration of the sewage (such as in cannery sewage),

(iii) Intermediate treatment between plain sedimentation and secondary biological treatment is required,

(iv) Sludge conditioning for dewatering is needed.

(v) During emergent situation it becomes necessary to increase the settling capacity of an overloaded plain sedimentation tank.

Advantages and Disadvantages of Chemical-Aided Sedimentation:

The advantages of chemical-aided sedimentation are as listed below:

(i) Where ordinary plain sedimentation is sufficient but low stream flow at times lowers the dilution factor and necessitates somewhat better treatment. Moreover, this process is flexible enough to meet changing conditions because chemical dosing may be carried out or discontinued as desired.

(ii) As there is forced sedimentation, it is more efficient and space required for treatment unit is less.

(iii) Degree of treatment is better than plain sedimentation because chemical-aided sedimentation results in greater reduction of suspended solids and BOD.

(iv) Many industrial sewages do not respond to biological treatment and others require very good clarification as a pre-treatment. In such cases this treatment is very useful.

(v) Colour in sewage may be removed by this process.

(vi) This process also removes phosphates and other nutrients from the sewage which may help in controlling eutrophication.

The disadvantages of chemical-aided sediment are as listed below:

(i) Biological treatment methods presently adopted for sewage treatment are more effective and complete in themselves and therefore chemical-aided sedimentation is not of much assistance. Moreover addition of chemicals makes the biological treatment processes more difficult.

(ii) Chemical coagulants, while reacting with sewage, kill some of those micro-organisms which are responsible for the digestion of sludge.

(iii) Operational cost is high due to high cost of chemicals.

(iv) Skilled hands are required for operation of these processes.

(v) Additional provision has to be made for handling and storage of chemicals.

(vi) Quantity of sludge produced is very high which require careful processing and disposal.