CBB 2082

PROCESS INSTRUMENTATION

GROUP ASSIGNMENT

STUDENT NAME ID.

EDDIE CHANG JEE TED

JEREMY MELVIN AMBOI 12595

THIAN JUN YI 12884

SUBMISSION DATE: 22ND APRIL 2011

Domestic Wastewater Treatment Process

Domestic wastewater treatment, or sometimes known as sewage treatment, is the process of removing contaminants from wastewater and household sewage. The main purpose of the wastewater treatment process is to produce an environmentally-safe fluid waste stream, as well as practice a proper and suitable disposal or reuse of solid waste.

The domestic wastewater treatment process can generally be summarized by the above figure. Five basic steps to a complete wastewater treatment process include a preliminary treatment, three core treatment processes and a solids processing phase.

Preliminary treatment

- the first stage of the wastewater treatment process- screens and removes larger inorganic material such as paper and plastics- removes particles such as grit and slit which are abrasive to plant

equipment

Primary treatment - wastewater is passed through a sedimentation tank- solid particles of organic material are removed from the suspension by

gravity settling- resultant sludge is concentrated and pumped away

Secondary treatment - wastewater enters an aeration tank where oxygen is provided to promote the growth of microorganisms

- these microorganisms consume organic pollutants and nutrients in the wastewater to grow and reproduce

- wastewater is then passed through a secondary sedimentation tank where the biomass settles under gravity and is concentrated as sludge

- the sludge can be recycled to the inlet of the aeration tank

Tertiary treatment - wastewater is disinfected using chlorine to reduce pathogens and other microorganisms

Solids processing - treatment of biodegradable sludge with anaerobic bacteria, which do not need oxygen for growth

- the methane gas produced in this process is used to heat the digesting sludge, and sometimes also used to generate electricity

- water is removed from the digested sludge through centrifugation or solar evaporation in lagoons

- the remaining bio-solids are then sent to landfills, incinerated or used as fertilizers

One of the many instruments used in the domestic wastewater treatment process is the butterfly valve.

The Butterfly Valve

The butterfly valve is widely used in chemical applications involving fluids, which include the domestic wastewater treatment process. It is used to control the flow of material through a circular pipe or tube, typically if the material is air, gas, steam or liquid.

Essentially, a butterfly valve is made up of a body, a seat, a stem, a disc and an actuator. The disc is positioned in the center of the valve, and a rod passes through the disc and connects to an actuator n the outside of the valve. By rotating the actuator, the disc is turned either parallel or perpendicular to the flow of fluid.

There are generally three main types of butterfly valve:

Inflatable Seated - inflatable seated butterfly valve uses an inflatable bladder to create a seal around the disc

- once the disc is closed, the bladder will inflate pneumatically, enabling less torque being required to move the disc

Double Offset - double offset butterfly valve is ideal for applications involving high temperatures and high pressures

- it can withstand temperature up to 1200°F and pressure up to 1440 psig

Triple Offset - triple offset butterfly valve is often used in refinery and offshore applications

- it can withstand extreme conditions and is fire safe

Advantages and Disadvantages

The advantages and disadvantages of butterfly valves are generally depending on the work condition, which involves factors like temperature, flow rate, working pressure, viscosity and volatility of fluid etc. But for comparison purposes, the common advantages and disadvantages of the butterfly valve are listed as followed:

Advantages Disadvantages

1. Lightweight, relatively small and has simple structure which is quick-acting

2. Economic – sold at cheaper price, has lower cost of installation and maintenance

3. Has high capacity for larger flow rate and lower pressure drop

4. Has replaceable seat, hence the valve seat does not require grinding, lapping or machine work

5. Resilient seat is held in place by mechanical means – no need for bonding or cementing

1. Not a good throttling valve as it creates turbulence once it is opened

2. Usually has to be either fully shut or fully open3. The normal operating pressure ranges tend to

be low or medium4. High torque is required to control the valve5. Precise flow control is difficult to achieve6. Cavitation may occur if flow rate of the fluid

passing through the valve is slow

Working Mechanism of the Valve

The ranges of working condition for a butterfly valve is listed as followed:

Pressure - 150-psi to 740-psi cold working pressure

Temperature - 25°F to 450°F

Size - Can be built in any sizes, depending on the requirement to fit a piping system

The butterfly valve belongs to a family of valves called the quarter-turn valves, whereby it only rotates 90° or less. The metal disc, which is commonly known as the ‘butterfly,’ can be controlled by a handle which is connected to a rod the metal disc is attached to. By turning the handle by a quarter, the disc is turned so that it completely blocks of the passageway, hence closing the valve. When the valve is fully open, the disc is rotated a quarter turn in the opposite direction so that it allows an almost unrestricted passage of the fluid. The valve may also be opened incrementally to throttle flow.

Different kinds of butterfly valves have different working principles to adapt to different pressures and usage. For example, the inflatable seated butterfly valve, which uses the flexibility of rubber, has the lowest pressure rating. The high performance butterfly valve (e.g. the triple offset butterfly valve) features a slight offset in the way the disc is positioned, which increases the valve’s sealing ability and decreases its tendency to wear.

Actuator used in Butterfly Valves

Actuators used in valves depend on the working consideration, i.e. size and application of the valve. In the case of butterfly valves, we use the part-turn actuators, which is an actuator that transmits torque to the valve for less than one full round. In this case, one whole round means a movement of less than 90O.

The diameter of the actuator ranges from a few inches to a couple of meters while the torque requirements to run the actuator is around 10 Nm and up to 100 00 Nm. In more modern equipment, electrical actuators are used compared to pneumatic actuators for conditions such as large valve diameter and high torque requirement.

The advantage of this actuator is purely due to its mechanical build that only requires a small rotation to open the valve, thus reducing amount of force and time needed. Due to the type of working environment in the wastewater industry, time is an essence to everyone since domestic waste comes in a large and frequent amount.

Other Instruments involved

In wastewater treatment, drying is also an important process in treating the water. High temperatures are needed to solidify for easiness to dispose the material. This sludge is dehydrated to reduce its volume and is channeled into an incinerator, injected into land or dispose in a landfill. Therefore, temperature measurement device is required to maintain the temperature of the dryer. The most suitable instrument for this process is a thermocouple.

Thermocouple is a device that contains a junction between two different metals that produces voltage based on temperature difference. Thermocouple is chosen as compared to other temperature measuring devices because it is commonly available and cheap.

The working principle of a thermocouple is based on metals having different thermal resistance, thus generating voltage. This is also known as thermoelectric effect or the Seebeck effect. Finally, we can obtain the temperature of the metal by measuring the voltage of the metal being heated.

Instrument Mechanism FunctionpH Controller Consists of acid discharging

device to releases acid upon activation to adjust the level of

pH in the wastewater

Maintain and adjust pH level of wastewater

Gas Detector Consists of a Wheatstone Bridge transducer (by working principle of gas detection through heat)

that senses these gases and signals it to the alarm system

Detect harmful gases that might accumulate in the plant i.e.

methane

Odor Controller Releases nitrate oxygen into the wastewater stream, causing microorganism to work on

dissolving and removing hydrogen sulphide from the

wastewater

Eliminates odor

Oxygen Controller Consists of an oxygen sensor to sense the level of oxygen

concentration

Controls the oxygen concentration in a reactor (to be

supplied to microorganism for aerobic respiration to degrade

organic matter)Optical Sensor Has lens that emits lasers to

measure the wavelength of the specified solid. Through

spectrophotometric process, one is able to obtain the

concentration through the acquired wavelength

Measure solid concentration

Pressure Sensor Has pressure indicator and detector to gather how much pressure is exerted onto the

device

Sense changes in pressure in piping system

References

SA Water. (2004, Feb). Retrieved 2011, from http://www.sawater.com.au/SAWater/Education/OurWastewaterSystems/Wastewater+Treatment+Process.htm

Abdullah, J. (n.d.). Biotek. Retrieved 2011, from http://www.biotek.gov.my/nbs2010/program/oral/ind/abstract/day2/Jaafar%20Abdullah%20SIRIM.pdf

RegulatoryStaff. (n.d.). Retrieved 2011, from http://www.regulatorystaff.sc.gov/orscontent.asp?pageid=654

Remy Valve. (n.d.). Retrieved 2011, from RMPV.com: http://www.rmpv.com/en_web/pro_valve/butterfly-valve.html

TPUB.com. (n.d.). Retrieved 2011, from http://www.tpub.com/fireman/69.htm

Wikipedia. (n.d.). Retrieved 2011, from Butterfly Valve, Valve Actuator, Sewage Sludge System, Thermocouple: http://www.wikipedia.org/