What is the principal objective of waste water management?

The principal objective of wastewater treatment is generally to allow sewage and industrial effluents to be disposed of without danger to human health or damage to the environment. Conventional wastewater treatment consists of a combination of physical, chemical and biological processes and operations to remove solids, organic matter and sometimes nutrients from wastewater. Wastewater treatment methods can be broadly classified as:

(i) Physical unit operation: The removal of pollutants by physical forces.

(ii) Chemical unit operation: The removal of pollutants by addition of chemicals or by chemical reactions.

(iii) Biological unit operation: The removal of pollutants by biological activities.

These treatment methods occur in a variety of combinations in wastewater treatment systems, to provide various levels of wastewater treatment. The typical flow diagram for wastewater treatment

There are generally four levels of wastewater treatment:

1) Preliminary treatment

2) Primary treatment

3) Secondary treatment

4) Tertiary/advanced treatment.

Preliminary Treatment

Preliminary treatment is the first step in wastewater treatment. The purpose of preliminary treatment is the removal of coarse solids and other large materials often found in wastewater.

Preliminary treatment consists mainly of physical unit operations such as:

1. Screening

Screening in the removal of coarse solids in wastewater which may clog the mechanical equipments and pipes bar racks and screens are common types of screening devices. Most screens in treatment plants consist of parallel bars placed at an angle in a channel in such a manner that the wastewater flows through the bars. Trash collects on the bars and is periodically raked off by hand or by mechanical means. In most places these screenings are disposed of by land filling or incineration.

2. Combination

Combination is the grinding of coarse solids into smaller and more uniform particles, which are then returned to the flow stream for subsequent treatment.

3. Flotation

Flotation in the separation of suspended and floatable solid particles from wastewater this can be achieved by introducing fine air bubbles into the wastewater.

4. Grit removal

Grit includes sand, ash, cinder, egg shells, etc. of diameter less than 0.2 mm. The specific gravity of grit varies from 2.0 to 2.6. Grit should be removed early in the treatment process because it is abrasive and rapidly wears out pumps and other equipments. Since it is mostly inorganic, it cannot be broken down by biological treatment processes and thus should be removed as soon as possible.

Grit is usually removed in a long narrow trough called a “grit channel” (grit channel is designed to provide a flow-through velocity of 0.3 m/s. The settled grit can be removed either manually or mechanically.

Primary Treatment

After preliminary treatment, wastewater is passed forward for primary treatment. The objective of primary treatment is the removal of settle able organic solids by sedimentation and the removal of materials that float (scum) by skimming.

Primary sedimentation tanks or clarifiers may be circular or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time (time taken by a particle to travel from inlet to outlet) ranging between 2 and 3 hours. In a circular basin, the flow pattern is radial. To achieve the radial flow pattern, the wastewater can be introduced in the centre of the basin. At the centre of the basin, the wastewater enters a circular well, designed to distribute the flow equally in all directions. The scraper attached to a rotating arm rotates slowly around the bottom of the tank. The scraper pushes the settled solids (sludge) towards the centre and into the sludge hopper. The settled solids are known as primary sludge. They are collected for further treatment prior to disposal.

Scum is collected by a rotating blade at the surface. The clear surface water of the primary tank flows from the tank by passing over a weir. The weir must be long enough to allow the treated water to leave at a low velocity; if it leaves at a high velocity, particles settling at the bottom may be picked up and carried from the tank.

Approximately 30% of the incoming biochemical oxygen demand (BOD₅), 50-70% of the total suspended solids (SS) and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen and phosphorus are also removed during primary sedimentation. The effluent from primary sedimentation units is called as the primary effluent.

Secondary Treatment (Biological Treatment)

The goal of all biological treatment systems is to remove the dissolved and the non- settling organic solids from the primary effluent by using microbial populations. Biological treatments are generally part of secondary treatment systems. The microorganisms used are responsible for the degradation of organic matter and the stabilisation of organic wastes. With regard to the way in which they utilise oxygen, they can be classified into:

1. Aerobic (require oxygen for their metabolism)

2. Anaerobic (grow in the absence of oxygen)

3. Facultative (can proliferate either in the presence or absence of oxygen).

Stabilization of organic matter by microorganisms in a natural or controlled environment of biological treatment process is accomplished by two distinct metabolic processes:

1. Respiration

2. Synthesis.

Respiration is a microbial process in which a portion of the available organic substrate is oxidized by microorganisms to liberate energy. The energy derived from respiration is utilized to synthesize new microbial cells.

The biological treatment processes used for wastewater treatment are broadly classified as aerobic (in the presence of oxygen) and aerobic (in the absence of oxygen).

Aerobic process

Aerobic degradation occurs in two steps. In the first step, complex organics (carbohydrates, proteins, lipids, etc.) are broken down by extracellular enzymes into simple organic compounds. In the second step aerobics microorganisms (in the presence of oxygen) convert simple organic compounds into oxidized end products such as carbon dioxide, nitrate and phosphate. The energy released in this process is used for biosynthesis of more bacterial cells

If the microorganisms are suspended in wastewater during treatment, the operation is called ‘suspended growth process’. When the microorganisms are attached to a surface over which they grow, the operation is called ‘attached growth process’. So the conversion of organic matter to gaseous end products and cell tissue (biomass) can be accomplished aerobically, an aerobically or facultative using suspended and attached growth systems.