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Design Philosophy of Wastewater Treatment Plant

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After we discussed about water treatment plant, I want to share my new task about designing wastewater treatment plant. The explanation in this post will be quite simple and not really deep because I also still learn the fundamentals of designing water treatment plant. But after I discussed with one wastewater treatment plant vendor, I got more understanding about the plant and basic consideration in designing what equipment or process required in wastewater treatment plant.

Classes of Wastewater Contaminants

Industrial wastewater treatment plant is mechanisms or processes used to treat water that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use (source). EPA (Environmental Protection Agency) has defined classes of wastewater contaminants.

  • Conventional pollutants. It is water pollutant that is amenable to treatment by a municipal sewage treatment plant. It includes BOD, TSS, oil and grease, fecal coliform bacteria, and pH
  • Toxic pollutants (priority pollutants). It includes 126 “priority pollutants”, heavy metals (e.g. Cu, Pb, Hg), and organic compounds (e.g. PCBs, dioxin)
  • Non-conventional pollutants. e.g. chlorine, ammonia, nitrogen, phosphorous.
  • VOCs (Volatile Organic Compounds). 

Typical Industrial Wastewater Contaminants

The characteristics of wastewater in industry can be different. It depends on what kind of industry.

Typical industrial wastewater contaminants

Outside diameter (mm)Wall thickness (mm)
161.21.62.0--
20-1.62.02.6-
25-1.62.02.63.2
30-1.62.02.63.2
38--2.02.63.2
50--2.02.63.2

(Source)

Problems Associated with Wastewater Pollutants

The components in wastewater can be a potential problem when it is not treated well. Table below shows several problems associated with wastewater pollutants.

Preferred length of tube
6 ft (1.83 m)
8 ft (2.44 m)
12 ft (3.66 m)
16 ft (4.88 m)
20 ft (6.10 m)
24 ft (7.32 m)

(Source)

Basic Design Consideration

There are five significant factors that are essential for the design of wastewater treatment (source).

  • Strength and characteristics of wastewater
  • Flow rates and their fluctuations
  • Mass loading
  • Design criteria
  • Hydraulic flow diagram

Strength and Characteristics of Wastewater

For the design of wastewater treatment plant, the first important information one should have is the strength and characteristics of wastewater. It is normally expressed in terms of pollution load, which is determined from concentration of physical, chemical, and biological contents in wastewater.

Physical characteristics of wastewater can be expressed by:

  • Solids: Total dissolved solid, total suspended solid, volatile and fixed or mineral solids
  • Color
  • Odor
  • Temperature

Chemical characteristics of wastewater can be expressed by:

  • Organic contents: BOD, COD, fats, phenols, surfactants, oil, grease, etc
  • Inorganic contents: alkalinity, chlorides, nitrogen, sulphur, phosphorous, heavy metals, pH, carbohydrates
  • Gases: Oxygen, methane, hydrogen sulfide

Biological characteristics can be expressed by:

  • Animals
  • Plants
  • Protista
  • Viruses

Understanding of these parameters is very necessary for selecting a wastewater treatment plant system and the amount of pollutants to be removed to a level that meets local environment regulatory.

Flow rates and their fluctuation

The next essential consideration in designing wastewater treatment plant is the quantity or volume of wastewater in terms of flow rates. It is the total of wastewater generated daily and to be treated everyday.

The flow of wastewater is never steady. The flow reaching maximum and minimum values (for example, the flow rate of wastewater increases due to boiler blow down, reverse osmosis cleaning, and mixed bed polisher regeneration). It will have significant effect on the size of pumping, treatment units, and flow conduits that have been designed on consideration of average flow rates.

Table below summarizes application of various flow rates in the design of a wastewater treatment plant.

Criteria30 deg triangular60o Rotated triangular
Arrangement
Heat transferHigh
Pressure dropHigh
ApplicationLess fouling fluid
Fixed tube sheet design
Tube accommodationAccommodate more tubes
Effect on shell sizeSmall shell size
LimitationLimited to clean shell side service
Ease on shell side mechanical cleaning-
PopularityPopularLess popular compared to 30 deg triangular

Mass loading

Mass pollution load is usually defined as the product of volume (flow rates) and strength of wastewater and is expressed as mass load per unit time. For example, wastewater having 1000 m3/day flow and 300 mg/L BOD has the mass pollution load of BOD equal to 300 kg/d. As the performance of wastewater treatment plant is influenced by the variation of flow rates, it is important to calculate ratios of peak to average and/or minimum mass load to check the design of treatment facilities.

Design Criteria

Design criteria is guideline values for designing new wastewater treatment facilities which is determined through research and laboratory scale model studies as well as operational experience. The most frequently assumed criteria for designing wastewater treatment plant are:

  • Detention period or time (Hydraulic Retention Time, HRT)
  • Flow through velocity (Horizontal velocity of flow)
  • Settling velocity (terminal velocity of settling particles)
  • Surface loading rates or overflow rates
  • Weir loading rates
  • Organic loading
  • Food to microorganism ratio
  • Mean cell residence time
  • Hydraulic loading
  • Volumetric loading
  • Basin geometry

Learn more.

Source of picture.

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