In today's scenario, where the supply of fresh water is gradually running dry, what can we do to avoid the impending catastrophe of running out of good quality water?

That is where these tips on wastewater treatment become pertinent, for your Institution, to manage wastewater treatment plants efficiently. Here I have explained how the pretreatment of water helps you increase the efficiency of your wastewater plant.

Pre-treatment involves the use of simple mechanical processes to remove rubbish at very low cost, so that the efficiency of the wastewater treatment plant is increased and cost is minimized.

The pre-treatment process can be carried out by means of: (a) Screens (b) Grit chambers (c) Skimming tanks (d) Grease traps

Let's take a look at each method, one at a time.

Screening

The first step in the treatment of sewage is to remove floating and suspended matter such as cloth, paper, kitchen refuse, pieces of wood, cork, hair, fiber, fecal solids, etc. This can be achieved through screening process which:

(i) Prevents clogging of sprinkler nozzles or the surface of trickling filters. (ii) protects pumping parts, siphons etc., from damage. (iii) improves the efficiency of the biological processes, as the floating solids occupy excessive space which ultimately reduce the retention time for wastewater. (iv) prevents floating matter in the receiving bodies of water. Next, let's see what is the geometry and dimension of the screen?

Screening is accomplished by means of screens, having openings of a uniform size, circular or rectangular in shape.

The screening element is comprised of parallel bars, rods or wires, grating, wire mesh or perforated plate. When composed of parallel bars or rods, it is called a rack or bar screen and when made from wire mesh, perforated plate etc, it is called screen. Screens may be classified further, depending upon the size of their openings, as coarse, medium and fine.

It is usual in sewage treatment to employ medium bar screens of opening 25 mm or more. Iron bars set on edge across the channel through which sewage flows with a velocity of at least 0.45 m/sec. The bars stop in the direction of flow, the angle with the horizontal being 30 to 60. This facilitates manual cleaning of screens by the upward stroke of the rake. Screenings are allowed to drain off for some time on a perforated platform over the channel.

Disposal may be through burial in trenches, incineration and disintegrating in shredders and returning to the sewage or passing to the sludge disposal plants.

Grit chambers

How to remove solids having specific gravity greater than water?

The solution lies with the use of a grit chamber. This is an enlarged channel or long basin in which the cross-section is increased to reduce the velocity of the flowing sewage sufficiently to cause heavy inorganic matter such as grit, sand and gravel of size 0.2 mm, and larger to settle, while the lighter organic matter remains in suspension.

Design factors

(i) Velocity of flow (ii) Period of detention, what are the design factors to be considered? The factors to be considered in grit chamber design are: Velocity of wastewater flow must be 0.3 m/sec. This will permit the deposition of the bulk of heavier mineral solids while most organic matter remains in suspension. A velocity of flow in the range 0.15 - 0.3m/sec. is generally recommended. In order to keep the velocity within the desirable limits, it is usually necessary to provide two or more channels to manage fluctuations in sewage flow. One minute (volume of the grit chamber/flow rate) is the detention time normally employed. Since sedimentation of granular solids is dependent to a large extent upon the surface area of the chambers, their width could be kept large. A length to width ratio of 8 to 1 may be used limiting the effective depth to about 2 m.

(iii) Method of cleaning

Grit chambers are cleaned by hand, mechanically or hydraulically. Hand cleaning is done only in the case of smaller plants, is less hygienic and odor-free though somewhat easier for disposing off the removal material than in the case of mechanical cleaning.

(iv) Grit storage space for grit may be provided throughout the length of the chambers or by means of one or more pits deeper than the remainder of the basins. Concentration of grit is also useful for cleaning purposes. Channel may be provided with a frequency of cleaning of 15 days.

Skimming tanks

How to remove the floating solids from wastewater?

A skimming tank is a chamber so arranged that the floating matter, oil, fat, grease etc., rise and remain on the surface of the sewage until removed, while the liquid flows out continuously under partitions or baffles. It is necessary to remove the floating matter from sewage otherwise it may appear in the form of unsightly scum on the surface of settling tanks or interface with the activated sludge process of sewage treatment. The chamber is a long trough-shaped structure divided into two or three lateral compartments by vertical baffle-walls having slots for a short distance below the sewage surface and permitting oil and grease to escape into stilling compartments. Blowing air into the sewage from diffusers placed in the bottom brings about the rise of floating matter. Sewage enters the tank from one end, flows through longitudinally and leaves out through a narrow inclined duct. A theoretical detention period of 3 minutes is enough. The floating matter can be removed by hand or mechanically.

Grease traps

Grease traps are designed with submerged inlet and bottom outlet. The traps must have sufficient capacity to permit the sewage to cool and grease to separate. Frequent cleaning through removable covers is essential for satisfactory operation.

Hope this piece of information provides you with all the practical tips that aids pre-treatment process in your wastewater treatment plant.

The above is an excerpt from the free newsletter on "All About Wastewater Treatment" published by Geostar Publishing & Services LLC.

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Richard Runion 6423, Woodbine Court, St. Louis, Missouri, 63109, USA

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Richard Runion President Geostar Publishing & Services LLC