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) | ||||
|---|---|---|---|---|---|
| 16 | 1.2 | 1.6 | 2.0 | - | - |
| 20 | - | 1.6 | 2.0 | 2.6 | - |
| 25 | - | 1.6 | 2.0 | 2.6 | 3.2 |
| 30 | - | 1.6 | 2.0 | 2.6 | 3.2 |
| 38 | - | - | 2.0 | 2.6 | 3.2 |
| 50 | - | - | 2.0 | 2.6 | 3.2 |
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) |
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.
| Criteria | 30 deg triangular | 60o Rotated triangular |
|---|---|---|
| Arrangement |  |  |
| Heat transfer | High | |
| Pressure drop | High | |
| Application | Less fouling fluid | |
| Fixed tube sheet design | ||
| Tube accommodation | Accommodate more tubes | |
| Effect on shell size | Small shell size | |
| Limitation | Limited to clean shell side service | |
| Ease on shell side mechanical cleaning | - | |
| Popularity | Popular | Less 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
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