SEWAGE TREATMENT OPERATIONS AND PROCESSES

1 Physical Unit Operations

In the physical unit operations physical forces are utilized in some units for the removal of solid contaminants. The physical unit operations are:

1. Screening
2. Mixing
3. Flocculation
4. Sedimentation
5. Floatation
6. Filtration

2  Chemical Unit Processes

In chemical unit processes the removal or conversion of some solids, is brought about by the addition of chemicals or by other reactions. Common examples of chemical unit processes are:

• Gas transfer by aeration,

• Adsorption, and

• Disinfection

3  Biological Unit Processes

In biological treatment processes the removal or conversion of organic solids is brought about by the biological activities. They remove colloidal or dissolved bio-degradable organic substances in wastewater. Organic substances are converted into gases that can escape to the atmosphere and as biological cell tissues that can be removed by settling.

STAGES OF TREATMENT

Types and nature of solids present in wastewater and unit operations/processes used for their removal

Type of solids	Nature of solids	Unit Operations/Process used for removal
Floating	Coarse Organic and inorganic	Coarse screen
	Fine organic and inorganic solids and algae	Microscreens
Suspended Solids	Settleble inorganic	Grit chamber
	Settleble organic	Primary sedimentation tank
	Non settleble organic	Biological process followed by secondary sedimentation tank
Dissolved solids	Organic	Biological process
	Inorganic	Reverse OsmosisRoot zone treatment with certain type of trees and aquatic plants

The wastewater treatment is achieved in three stages; primary, secondary and tertiary treatments.. Some of the unit operations and processes are included in each stage of treatment. The process flow diagram of a typical wastewater treatment plant is furnished in  Table

1 Primary Treatment

The first stage of treatment is known as primary treatment which also includes certain preliminary operations such as flow equalization, comminution (or grinding), grease removal, flow measurement, etc. The unit operations in primary treatment are screening to remove larger floating objects, grit removal for removing inert sand and inorganic particles, and settling for removing settleable suspended organic solids. The main purpose of the primary treatment is to produce a generally homogeneous liquid capable of being treated biologically and a sludge that can be separately treated or processed.

2 Secondary Treatment

The next stage of treatment is secondary treatment, which is designed to remove soluble organics from the wastewater. Secondary treatment consists of a biological process and secondary settling. Secondary treatment is designed to substantially degrade the biological content of the sewage such as derived from human waste, food waste, soaps and detergent.

3 Types of Biological Treatment based on Process

There are two types of biological treatment process; aerobic and anaerobic. Aerobic process means that dissolved oxygen (DO) is present for the microbes for respiration. Anaerobic process means that the process proceeds in the absence of DO.

The effluent from primary treatment units is further treated generally using aerobic biological processes. For these processes to be effective, the microorganisms require both dissolved oxygen and a substrate on which to live. Oxygen can be supplied either through natural process or artificial mechanical means. In both cases, the bacteria and protozoa consume biodegradable soluble organic contaminants and bind much of the less soluble fractions into floc particles. The oxidization of organic substances can be achieved by anaerobic process also by anaerobic organisms, which don’t need DO. They take their oxygen requirement from complex organic substances, such as sulphate (SO₄), phosphate (PO₄), etc.

The end-products of aerobic and anaerobic processes are different. Under aerobic conditions, if completely oxidized, organic matter is transformed into non-hazardous products such as CO₂ and H₂O and cell tissues. But an anaerobic process, apart from CO₂ and H₂0 and cell tissues, can also produce methane (CH,), which is explosive, and ammonia (NH₃) and hydrogen sulfide (H₂S), which are toxic. Some materials are better degraded under anaerobic conditions than under aerobic conditions. In some cases, the combination of anaerobic and aerobic systems in a series provides better and more economical treatment than either system could alone provide.

Classification of biological treatment based on growth system

Biological treatment systems are classified into systems: (a) attached growth system, and (b) suspended growth systems. Aerobic and anaerobic biological systems are available in both attached and suspended growth configurations. Examples are as follows:

• Aerobic attached growth systems: trickling filter and RBC;

• Aerobic suspended growth systems: activated sludge process, aerated lagoon, waste stabilization ponds etc.

• Anaerobic attached growth systems: anaerobic filters and upflow anaerobic sludge blanket units.

• Anaerobic suspended growth systems: septic tanks and anaerobic ponds.

• The final step in the secondary treatment stage is to settle out the biomass as biological floc or filter material generated during biological treatment and produce sewage water containing very low levels of organic material and suspended matter. This settled biomass or secondary sludge is then piped to sludge-management systems, while the effluent from SST is disposed.

Sludge treatment and disposal

The sludge accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. The purpose of sludge digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. The choice of a wastewater sludge treatment method depends on the amount of solids generated and other site- specific conditions. However, in general, composting is most often applied to smaller-scale applications followed by aerobic digestion and then lastly anaerobic digestion for the larger-scale municipal applications.

4 DISINFECTION

The purpose of disinfection in the treatment of wastewater is to substantially reduce the number of living organisms in the water to be discharged back into the environment. The effectiveness of disinfection depends on the quality of the water being treated (e.g., TSS, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Turbid water will be treated less successfully since solid matter can shield organisms, especially from ultraviolet light or if contact time is low. Generally, short contact times, low doses and high flows are against effective disinfection. Common methods of disinfection include use of ozone, chlorine, or UV light. Chloramine, which is used for drinking water, is not used in waste water treatment because of its persistence. Disinfection follows secondary clarification in most treatment plants or after tertiary treatment when the wastewater reclamation and reuse is contemplated. Disinfection is normally accomplished with chlorine. Due to the potential environmental impact of chlorine, most plants now dechlorinate wastewater effluents before discharge.

5 TERTIARY TREATMENT

Third stage treatment is referred to as tertiary treatment or advanced treatment. More commonly used advanced systems are adsorption to activated carbon, filtration through sand and other media, ion exchange, various membrane processes, nitrification-denitrification, coagulation-flocculation, and micro-screening.