A few weeks ago I got a discussion about wastewater treatment system in our proposed plant. I discussed with one of my colleague. He said, “Let the nature works! Let it process the wastewater by itself”. It sounds weird at the first time. But he’s right and he’s a bit wrong.
He’s right because the natural water in streams, rivers, lakes, and reservoir have a natural waste assimilative capacity to remove solids, organic matter, even toxic chemicals in wastewater. He’s a bit wrong because the process takes a really long time. That’s why we need wastewater treatment system to make it quick (read more in Water and Wastewater Calculation Manual). Read More
As already explained in previous post, 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. In this post I want to discuss more about the design criteria of wastewater treatment plant. It is important to note that the design calculations are usually based on certain assumptions. Hence, it is important to have a clear understanding about the concept of design criteria and the significance in determining capacities and dimensions and wastewater treatment units.
Detention Time (Hydraulic Retention Time, HTR)
Detention time or hydraulic retention time is the average time spent by the influent sewage in the aeration tank. It is calculated as tank volume (m3) divided by flow rate.
Clearly the higher inflow rate Q, the sooner the sewage influent will reach the outlet and therefore the lower the residence time or hydraulic retention time will be.
In designing wastewater treatment plant, hydraulic retention time must be sufficiently long to remove the requisite proportion of BOD from the untreated wastewater. In a conventional activated sludge system, the HRT will be between 5 and 14 hours (source).
Hydraulic residence times for primary settling tanks (tanks used in the treatment using flocculent settling) are from 1.5 to 2.5 hours. Design considerations should include effects of lowflow periods to ensure that longer residence times will not cause septic conditions. Septic conditions increase potential odors, solubilization, and loading to downstream processes (source).
Flow Through Velocity or Horizontal Velocity
The solid particles in wastewater experience two types of velocities in any tank where the flow is continuous, one along horizontal direction due to drag force and another along the vertical direction due to gravitational force. The horizontal velocity is called flow through velocity, while the vertical is called settling velocity.
Flow through velocity is the velocity with which the wastewater flows through a treatment unit. It is equal to flowrate divided by cross sectional area.
In practice, the linear flow through velocity has been limited to 1.2 to 1.5 m/min for settling tank to avoid resuspension of settled solids (source).
Settling Velocity
Understanding of particle settling velocity is used to determine the depth of treatment unit to separate suspended solid by gravity settling and to check the adequacy of length or diameter of a tank to remove particles before the effluent flows out of basin. A sand particle of 0.2mm size with specific gravity of 2.65 is observed to settle at a rate of 2.3 cm/s (source).
Surface Loading Rate or Overflow Rate
Surface loading rate or overflow rate is the volume of wastewater (flow rate) applied per unit surface area of the treatment basin and is normally expressed in m3/d/m2. This is a significant design criterion used to determine the surface area of tank. As surface loading rate is the hydraulic flow applied per unit surface area of the tank, it is also known as hydraulic loading.
Typical values of surface loading rates for primary clarifiers are 600 to 1000 gpd/ft2 (source).
Weir Loading Rate
Weir loading rate or weir overflow rate is defined as the quantity of wastewater flowing over a unit of weir length of the tank in a day.
Weir loading rate = flow rate/length of wire
The maximum weir loading rate, to limit the influence of drawdown currents, is preferably about 300 m3/dm (source).
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 reuse (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)
Nonconventional 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.
The components in wastewater can be a potential problem when it is not treated well. Table below shows several problems associated with wastewater pollutants.
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:
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)