ENVE422: Wastewater Treatment Process Design Dr. Revanuru Subramanyam Department of Civil and Environmental Engineering University of Nizwa

Sewage/ Wastewater – is essentially the water supply of the community after it has been fouled by a variety of uses. From the standpoint of sources of generation, wastewater may be defined as a combination of the liquid (or water) carrying wastes removed from residences, institutions, and commercial and industrial establishments, together with such groundwater, surface water, and storm water as may be present.

Generally, the wastewater discharged from domestic premises like residences, institutions, and commercial establishments is termed as “Sewage / Community wastewater”. It comprises of 99.9% water and 0.1% solids and is organic because it consists of carbon compounds like human waste, paper, vegetable matter etc. Besides community wastewater / sewage, there is industrial wastewater . Many industrial wastes are also organic in composition and can be treated physico-chemically and/or by micro-organisms in the same way as sewage.

Wastewater treatment involves breakdown of complex organic compounds in the wastewater into simpler compounds that are stable and nuisance-free, either physico-chemically and/or by using micro-organisms (biological treatment).

The adverse environmental impact of allowing untreated wastewater to be discharged in groundwater or surface water bodies and/ or land are as follows:

Before the late 1800s, the general means of disposing human excrement was the outdoor privy while the major proportion of the population used to go for open defecation. Sewage treatment systems were introduced in cities after Louis Pasteur and other scientists showed that sewage borne bacteria were responsible for many infectious diseases. The Early attempts, in the 1900s, sewage treatment usually consisted of acquiring large farms and spreading the sewage over the land, where it decayed under the action of micro-organisms. It was soon found that the land became 'sick'. Later attempts included the discharge of wastewater directly into the water bodies, but it resulted in significant deterioration of the water quality of such bodies. These attempts relied heavily on the self-cleansing capacities of land and water bodies and it was soon realized that nature couldn’t act as an indefinite sink.

Therefore, other methods of treatment were developed to accelerate the forces of nature under controlled conditions in treatment facilities of comparatively smaller size.

The contaminants in wastewater are removed by physical, chemical, and biological means. The individual methods usually are classified as physical unit operations, chemical unit processes, and biological unit processes, and these operations and processes occur in a variety of combinations in treatment systems. Generally unit operations involve contaminant removal by physical forces, while unit processes involve biological and/or chemical reactions.

The most important factors that should be borne in the mind before the selection and design of any sewage/ wastewater treatment system are:

Reactors Wastewater treatment involving physical unit operations and chemical and biological unit processes is carried out in vessels or tanks commonly known as reactors.

Principal applications of reactor types used for wastewater treatment

Factors must be considered in the selection of the type of reactor Nature of the wastewater to be treated Nature of the reactions( homogeneous or heterogeneous) Reaction Kinetics governing the treatment process Process performance requirements, and Local environmental conditions.

Flow measurement Wastewater flow measurement is an important adjunct to wastewater treatment. A knowledge of hydraulic loading rates is necessary for the operation of many of the reactors in a wastewater treatment plant. Chemical additives, air volume, recirculation rates and many other operating parameters depend upon the hydraulic flow rate. The most common devices used for measuring flows in a wastewater treatment plant are (1) Parshal flumes and (2) Palmer-Bowlus flumes. These devices are essentially open-channel venturimeters have an established have an established flow-head relationship, from which flow is determined by simply measuring the water elevation at a given point.

What is Parshal flume and who invented it A Parshall flume has a special shaped open channel flow section which may be installed in a ditch ,canal, or lateral to measure the flow rate. The Parshall flume is a particular form of venturi flume and is named for its principal developer, the late Mr. Ralph L. Parshall.

Palmer-Bowlus flumes

Physical Unit Operations The physical unit operations most commonly used in wastewater treatment include (1) Screening (2) Flow equalization (3) Coarse solids reduction (Communition) (4) Mixing and Flocculation (5) Grit removal (6) Sedimentation (7) Filtration and (8) Flotation

Screening The first unit operation generally encountered in wastewater treatment plant is screening. A screen is a device with openings , generally uniform size that is said to retain solids found in the influent wastewater to the treatment plant. The principal role of screening is to remove coarse materials from the flow stream that could damage subsequent process equipment. Fine screens are sometimes used in place of or following coarse screens where greater removals of solids are required to protect process equipment.

Classification of Screens Note: Coarse and Fine screens are used in preliminary treatment of wastewater. Micro screens are removed in using fine solids from treated effluents.

Location : Should be installed ahead of the grit chamber. Approach Velocity: Velocity of approach is limited to approximately 0.45 m/s to provide screen area. Headloss through the screens: Headloss through mechanically cleaned coarse screens is limited to about 150 mm. The headloss through the coarse screen can be estimated using the following equation

FLOW EQUALIZATION

Large land area may be required. Advantages Helps in improving the performance of down- stream operations and reduces the operating & capital cost of down- stream process. Biological treatment is enhanced because of elimination of shock load due to flow rate & pollution load. Thickner/ settler and filter performance gets enhanced and their required surface area gets reduced. Disadvantages Large land area may be required. Additional capital and operating cost may be required. May cause odor problem for nearby residential colonies.  

Solution Please refer to https://dl. dropboxusercontent