Introduction: Wastewater sources and characteristic

 

Sources Of waste water

Broadly, the wastewater sources are categorize into agricultural, domestic and industrial sector. There is another sector which few people considered is actually the storm water which is the just rainwater which is coming and falling on the surface and generatingrunoff. So, urban runoff or storm water that matters so that is also a type of or a source of wastewater which can be considered. Although normally storm water, there are two opinions of considering storm water as a wastewater or not because it is rain water directly. So, it is not been used or it is not that way have been utilized for any purpose, but when it falls on the surface it acquires some pollutants and the characteristic of water is not usable without adequate treatment for many purpose, ok.

·         What are the characteristics of wastewater?

Wastewater physical parameters

• Color: Fresh wastewater is normally brown and yellowish, but over time it turns black.
• Suspended solids: these are insoluble solids suspended in a liquid and visible to the naked eye
• Temperature: For wastewater, it is correlated to the outside temperature while being warmer because almost nobody takes a cold shower
• Turbidity: Due to suspended solids, wastewater will have higher turbidity.

Chemical characteristics of wastewater

Wastewater contains different chemicals in various forms, as shown below.

 

• Chemical oxygen demand (COD): This is a measure of the amount of organic matter in wastewater based on the oxygen required to oxidize it.
• Nitrogen: It is measured in its different forms: nitrite, nitrate, ammonia, and organic nitrogen (which is the amount of nitrogen present in organic compounds)
• Phosphorus: It is generally measured in its mineral and organic form, total phosphorus
• Chlorine (Cl-)
• Sulfates (SO4-2)
• Heavy metals
• BOD
• D.O.

1. Dissolved oxygen

 

To maintain life in an aquatic environment, it is essential to maintain a sufficient level of oxygen. Indeed, this last one is part of one of the necessary parameters for the continuity of life and its evolution. It is essential for photosynthesis and the alteration of organic components.

The more the water is exposed to the air, the more it is stirred, and the more it is supersaturated with oxygen. However, when there is an excess of soluble organic matter, it is considered undersaturated. Indeed, these organic materials serve as food for many micro-organisms. These micro-organisms consume a lot of oxygen to develop and degrade this pollution. This explains the lack of oxygen in the wastewater. The temperature also affects this parameter. The colder it is, the more oxygen is soluble in water.

In general, it is the analysis of the dissolved oxygen concentration. It is measured with an oxymeter.

 

 

2.  Chemical Oxygen Demand COD

Chemical oxygen demand (COD) is a measure of all oxygen consuming substances. It is about :

• those that can be eliminated by wastewater treatment
• those that are not suitable for biological treatment.

This measurement of the amount of oxygen consumed by a water sample is performed with strong oxidizing reagents. For example, potassium dichromate can be used for this measurement. This parameter is expressed as the mass of oxygen consumed in relation to the sample volume. Practically, the measurement of oxidation is done by a COD test to quantify the amount of oxidizable material. The amount of reagent consumed for the oxidation of the organic matter present, reported in mgO2/L, corresponds to the COD.

COD is useful in terms of water quality because it allows :

• determine the effect of an effluent on the receiving environment
• determine the biochemical oxygen demand (BOD).

3. Biochemical oxygen demand BOD

 

BOD is a measure of the amount of oxygen required to remove or alter biologically degradable organic matter in wastewater.

The water sample is stored for five days at 20°C, without light and covered tightly. We talk about BOD ₅ because the analysis is done over 5 days. Some countries use other varieties such as BOD ₇ or BOD ₂₁, called ultimate BOD.

However, BOD₅ is mostly used around the world. The darkness prevents the risk of photosynthesis and the temperature of 20°c favors the propagation of micro-organisms fond of O2.

The study involves 2 samples:

• The first one will be used to know the initial quantity in O2
• The second will be used to measure BOD at the end of the study time

The degradation of organic pollutants by micro-organisms, or self-purification, consumes oxygen. It is this decrease in oxygen in the environment that is measured by the BOD₅.

Like the COD, the BOD ₅ is also expressed in mg/l of oxygen (mgO2/L). It allows to determine the impact of an effluent on the receiving environment.

Indeed, the BOD ₅ represents the proportion of organic matter that is naturally biodegradable, and therefore mobilizes oxygen in the waterways.

 

 

 

 

 

 4. Suspended solids

 

Suspended solids (SS) are the materials in the transient phase in wastewater treatment plants. That is, they are not in colloidal or dissolved form. As the name suggests, these are particles suspended in the liquid. They can be filtered and are composed of organic and mineral particles. TSS is a commonly used term although it is actually Total Suspended Solids (TSS).

TSS analysis consists of passing a volume of sample through a membrane filter. This membrane will then be placed in an oven at 105°C for at least one hour. The difference in weight before/after filtration is used to determine the amount of suspended solids. This is measured in mg/l.

TSS is one of the parameters commonly used to determine the quality of a wastewater because it represents a danger to the receiving environment.

 

 

 

 

 

5. Total nitrogen Kjeldhal NTK or NK:

 

Domestic wastewater is made up almost exclusively of organic nitrogen (Norg) and ammoniacal nitrogen (NH4+). This is generally the case for industrial waters, although there is a wide variation in input nitrogen values from one company to another.

Organic nitrogen is a component of living cells (amino acids, proteins) while ammoniacal nitrogen NH4+ comes from :

• direct effluents from living beings (urine)
• from the decomposition of organic nitrogen by micro-organisms.

The ratio between Norg and NH4+ is determined by the length of the collection network. The longer the time spent in the sewer line, the more microorganisms have time to transform the organic nitrogen into NH4+.

The Kjeldahl nitrogen parameter NTK represents the sum of ammoniacal and organic nitrogen in the water expressed in mg/L. This is a complicated analysis to perform, so it is usually calculated as follows:

NTK = total nitrogen NGL – nitrite NO2 – nitrate NO3

For domestic wastewater, nitrates and nitrites are almost non-existent. Thus it is common practice to do only a total NGL analysis and to consider that NTK = NGL.

When a high concentration of Kjeldahl nitrogen is detected in a river, it indicates pollution of human origin. Organic nitrogen must be removed because it significantly reduces the oxygen concentration of an environment. This is why discharge standards are often strict for this parameter, and even more so when the receiving environment is considered a sensitive area.

It is historically named after the Danish chemist who discovered the method in 1883.

 

Reactors used for treatment of water & wastewater

 

1. A: Batch Reactor

 

In this type of reactor, the wastewater flow is not continuous and these reactors are operated as fill and draw type. The mode of operation of these reactors is batch mode with fill time, reaction time, and withdrawal time. The content may be completely mixed to ensure that no temperature or concentration gradient exists. All the elements in the reactor, under batch mode of operation, are exposed to treatment for the same duration of time for which the substrate is held in the reactor.

 

2. B:Plug-Flow (tubular flow) Reactor

 

In this type of reactor, the fluid particles flow through the tank and are discharged in the same sequence in which they enter the tank. The particles remain in the tank for a time same to theoretical detention time. No overtaking; no intermixing or dispersion takes place. Minimum longitudinal dispersion is considered and this type can occur in high length to width ratio of the tanks.

 

C:Continuous-flow Stirred Tank (Complete – mixed) reactor

 

In this reactor type, particles are dispersed throughout the tank as they enter the tank. The content in the reactor are completely mixed thus are homogeneous at all points in the reactor. This reactor can be made in square, circular or rectangular shape. The particles exit the tank in proportion to their statistical population.

 

 

4. D: Arbitrary Flow

 

Partial mixing between plug flow and completely mixing condition exists in this reactor. Each element of the incoming flow remains in the reactor for different duration of time. It is also called as dispersed flow and lies between ideal plug flow and ideal completely mixed reactor.

 

5. E: Packed Bed Reactor

 

Packing medium, such as, rock, slag, ceramic or synthetic plastic media are filled in these reactors. They can be anaerobic filter, when no air is supplied, or aerobic (trickling filter) when flow is intermittent or submerged aerobic filter when compressed air is supplied from the bottom.

 

6. F: Fluidized Bed Reactor

 

This reactor is similar to packed bed reactor except packing medium is expanded by upward movement of fluid (or air) than resting on each other in fixed bed. The porosity can be controlled by controlling flow rate of wastewater.

 

Types of unit Processes & operations

 

 

·         BIOLOGICAL UNIT PROCESSES:

Treatment methods in which the removal of contaminants is brought about by biological activity are known as biological unit processes.

· This is primarily used to remove biodegradable organic substances from the wastewater, either in colloidal or dissolved form.

· In the biological unit process, organic matter is converted into gases that can escape to the atmosphere and into bacterial cells, which can be removed by settling.

· Biological treatment is also used for nitrogen removal and for phosphorous and sulphate removal from the wastewater.

The different treatment methods used in wastewater treatment plant are classified in three different categories as:

· Primary Treatment : Refers to physical unit operations.

· Secondary Treatment: Refers to chemical and biological unit processes.

· Tertiary Treatment: Referes to any one or combination of two or all three i.e., physical unit operations and chemical or biological unit processes, used after secondary treatment.

Conventional water & wastewater treatment units

 

While there are many types of conventional sewage

treatment systems, four of the most common systems are

presented below. Each of the following systems can treat

sewage to meet government discharge regulations in South

and Southeast Asia.

1. Activated sludge

2. Sequencing batch reactor

3. Trickling filter

4. Rotating biological contactor

*  Activated Sludge :

First developed in England in the early 1900s, activated sludge has commonly been used worldwide to treat large volumes of wastewater. By definition, activated sludge refers to “the active population of micro-organisms that are used to treat wastewater.” Through a series of controls, pumps, and tanks, the sewage undergoes various processes in separate basins or tanks; these processes include: primary settling, aerobic biological treatment, secondary settling, disinfection, and discharge.

* Sequencing Batch Reactor :

The sequencing batch reactor (SBR) is a “fill and draw” variation on the activated sludge process. Instead of using separate tanks for each process, the entire treatment process occurs in one tank.

 The SBR processes are:

 • Fill – the tank is filled with wastewater;

 • React – the wastewater is aerated and mixed with biomass that has been acclimated to the wastewater environment in previous cycles;

 • Settle – the aeration is turned off and the reacted wastewater is allowed to settle; and

 • Decant – the treated and clarified supernatant (water in the upper portions of the tank) is pumped out to be disinfected and then discharged.

 

               

3. Trickling Filters:

 After wastewater passes through a settling tank, it flows through a tank filled with media, such as crushed rock. As the wastewater passes through the media, bacteria living on the media consume the organic material in the wastewater as food.

Trickling filters are simple devices that can operate without electricity. The force of the water is usually all that is necessary to move the distribution arm. A fixed spray device can also distribute the pre-treated wastewater.

Trickling filters are very efficient in treating wastewater. Although they still require primary settling basins, clarifying and disinfection units, they use less energy than activated sludge plants.

4. Rotating Biological Contactor

 The rotating biological contactor (RBC) system is a media system similar to the trickling filter; however, the media is on a disc that slowly rotates through the wastewater.

At the center of the RBC is a solid steel shaft connected to a drive unit and small motor. As the disc rotates through the settled sewage, the bacteria living on the media consume the organic material in the wastewater. RBC units can be very effective in treating wastewater.

 As the disk rotates through the air, oxygen transfer occurs with minimum energy input

5. Disinfection

Disinfection is required to kill the pathogens that might remain in the treated wastewater. Chlorine disinfection is the most popular method, although ozone and ultra violet (UV) light are becoming popular. If chlorine is the disinfectant of choice, it must be in contact with the wastewater for a minimum of 15 minutes before the water is discharged into the environment. UV lights and/or ozone generators may be incorporated into treatment systems using specialized equipment.