ENVE422: Wastewater Treatment Process Design

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 sufficient 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

Coagulation & Flocculation

About Coagulation & Flocculation In wastewater treatment operations, the processes of coagulation and flocculation are employed to separate suspended solids from water.  Although the terms coagulation and flocculation are often used interchangeably, or the single term "flocculation" is used to describe both; they are, in fact, two distinct processes. 

Purpose of Coagulation & Flocculation in Wastewater treatment Finely dispersed solids (colloids) suspended in wastewaters are stabilized by negative electric charges on their surfaces, causing them to repel each other.  Since this prevents these charged particles from colliding to form larger masses, called flocs, they do not settle.  To assist in the removal of colloidal particles from suspension, chemical coagulation and flocculation are required. 

Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart.  Cationic coagulants provide positive electric charges to reduce the negative charge of the colloids.  As a result, the particles collide to form larger particles (flocs).  Positively charged coagulants attract to negatively charged particles due to electricity Negatively charged particles repel each other due to electricity

Neutrally charged particles attract due to van der Waal's forces

Particles and coagulants join together into floc ► Flocculation  During flocculation, a process of gentle mixing brings the fine particles formed by coagulation into contact with each other.   Particles and coagulants join together into floc ►  

Perikinetic and Orthokinetic Flocculation The flocculation process can be broadly classified into two types, perikinetic and orthokinetic . Perikinetic flocculation refers to flocculation (contact or collisions of colloidal particles) due to Brownian motion of colloidal particles. The random motion of colloidal particles results from their rapid and random bombardment by the molecules of the fluid. Note: Brownian Motion Any minute particle suspended in a liquid (or gas) moves chaotically under the action of collisions with surrounding molecules

Orthokinetic flocculation Orthokinetic flocculation refers to contacts or collisions of colloidal particles resulting from bulk fluid motion, such as stirring. Flocculator

Coagulant Chemicals Types of Coagulants Coagulant chemicals come in two main types - primary coagulants and coagulant aids.  Primary coagulants neutralize the electrical charges of particles in the water which causes the particles to clump together.  Coagulant aids add density to slow-settling flocs and add toughness to the flocs so that they will not break up during the mixing and settling processes.  Primary coagulants are always used in the coagulation/flocculation process.  Coagulant aids, in contrast, are not always required and are generally used to reduce flocculation time. 

Commonly Used Coagulants and Coagulant aids Chemical Name Chemical Formula Primary Coagulant Coagulant Aid Aluminum sulfate (Alum) Al2(SO4)3 · 14 H2O X Ferrous sulfate FeSO4 · 7 H2O Ferric sulfate Fe2(SO4)3 · 9 H2O Ferric chloride FeCl3 · 6 H2O Cationic polymer Various Calcium hydroxide (Lime) Ca(OH)2 X* Calcium oxide (Quicklime) CaO Sodium aluminate Na2Al2O4 Bentonite Clay Calcium carbonate CaCO3 Sodium silicate Na2SiO3 Anionic polymer Nonionic polymer