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Biological processes for wastewater treatment

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Biological processes for wastewater treatment

Biological treatment of wastewater collected from residences, industries, commercial sectors, and agriculture is required to protect water quality through the removal of dissolved biodegradable organics, suspended organic and inorganic solids, nitrogen, and phosphorous as well as toxic substances like heavy metals and chemicals. There are 4 treatment levels of wastewater including pre-treatment, primary treatment, secondary, advanced and sludge treatment.

Pretreatment is generally required for protecting wastewater treatment plants by removing debris, grit, FOG (Fats – Oil – Grease) and large objects. Course and fine bar screens are required for pretreatment. Primary treatment generally removes the settleable solids (that are not removed in preliminary treatment) and BOD. The secondary treatment process is divided into aerobic and anaerobic processes, and it removes BOD, COD, and partially removes nutrients, and pathogens. Advanced treatment is used after the conventional secondary treatment process and removes organics, inorganics (calcium, potassium, nitrate, phosphorous) and specific toxic components. The sludge treatment is required for volume reduction, safe disposal of sludge, and its thickening and digestion.

Overview of wastewater treatment


Schematic of wastewater treatment plant

Preliminary treatment

The preliminary treatment is essential for the removal of untreatable course materials, protection, and improvement of the performance of subsequent treatment units[1]. Preliminary treatment unit operations include bar screens, screw pumps, flow measurement, shredders or grinders, grit removal, and flow equalization.

Convectional arrangement for gravity flow combined preliminary process[2]

Bar racks and screens

Bar screens are used for large solid objects as rags, sticks, can, and plastics from wastewater. These generally include two screens with one handling peak hydraulic flow rate and the other as a standby. The approach velocity must be more than 0.4 m/s to avoid settling in the channel and less than 0.9 m/s to prevent the forcing of debris through the screen[3].

Pump station

A preliminary treatment system uses a pumping station which elevates the wastewater to the desired level so that wastewater can flow to primary unit by gravity.

Flow measurement

The two most common flow meters used for wastewater are the parshall flume and the magnetic flow meter.

Parshall flume

In the design of the parshall flume, flow transition from subcritical to supercritical is caused by the transition of narrow to throat dimensions.

Magnetic flow meter

The magnetic flow meter uses faraday’s law theoretical basis for the operation. “When an electrical conductor passes through an electromagnetic field, an electromotive force or voltage is induced in the conductor that is proportional to the velocity of the conductor. The voltage thus generated is mutually perpendicular to both the velocity of the conductor and the magnetic field”[4]

Faraday's law as displayed in equation 20-2 [5]
Schematic of magnetic flow meter[6]

Coarse solids reduction

To avoid capturing coarse solids on barracks and/or screens a mechanical device is used to shred or grind the solids. Coarse solids should be removed from the wastewater early in the flow scheme to eliminate downstream problems.

Grit removal

This process removes the grit (sand, gravel, broken glass) by providing sufficient retention time and flow velocity to settle particles in the chamber.

Primary treatment

The basic principle of primary treatment is to remove the organic content like TSS (Total suspended solids), BOD (Biochemical oxygen demand), grease, oil, plastics, etc. by allowing particles to settle in a primary sedimentation tank.  As particles settle, they agglomerate, and their mass increases resulting in an increase in settling velocity. Total TSS removal and terminal settling velocity variation with time are design criteria for settling tanks. Detention time, surface overflow rate, weir loading rate and scour velocity are the main design parameters for the primary settling tank. These tanks are generally designed in rectangular or circular tanks.

Detention time

It is the theoretical time required for wastewater to pass through a tank for a specific flowrate.

“The rate of flocculation of particles by fluid motion (orthogenetic flocculation) may be described as first order with respect to the concentration of particles, the velocity gradient of the fluid motion, and the floc volume fraction” [7]

As shown in O'melia, 1972 figure 21-1 [8]

Scour velocity

Scour velocity is the horizontal velocity of the particles through the tank, which should be kept minimum to avoid resuspension (scour) of the settled particles back to the flow stream. The critical scour velocity may be estimated by the camp’s equation. [9]

Camp's equation as displayed in equation 21-2 [10]

Secondary treatment

The secondary treatment provides the further removal of BOD (80-90%) that escapes from the primary treatment and suspended solids (80-90%). It partially removes the nutrients (Nitrogen and phosphorus) and most of all pathogens. Nutrients have adverse effects on downstream units, if not removed in secondary treatment. [11]

The process is divided into aerobic and anaerobic processes. The aerobic processes utilize oxygen to decompose organics into carbon dioxide and water in presence of aerobic bacteria and release heat energy. The anaerobic processes utilize anaerobic bacteria that degrade organic compounds into carbon dioxide, water and methane in wastewater.

Activated slug process (ASP)

When wastewater flows through a suspended growth of biological solids (biomass) under aerobic conditions, the organic content is substantially reduced. The term “activated” refers to the return of active biomass from the secondary clarifier to the aeration basin.

In the conventional aerobic oxidation process, wastewater flows continuously into an aeration tank where air is injected to mix the activated sludge with the wastewater and to supply the oxygen needed for the organisms to oxidize the organic compounds.

Attached growth processes

Attached growth processes are aerobic processes, where microorganisms are attached to an inert surface called packing material and in the presence of air (provided either by natural draft or blowers), decomposition takes place. Attached growth processes are further divided into non submerged attached growth aerobic processes, partially submerged attached growth aerobic processes, sequential non-submerged attached growth aerobic processes, submerged attached growth aerobic processes, and activated sludge processes with biofilm carries.

Trickling filters

A trickling filter is an example of a non-submerged attached growth aerobic process that utilizes biological reactors using rock or plastic media. The wastewater is distributed on the surface of rock and plastic media. The biomass degrades the wastewater as it trickles down through the filter bed. Sloughing biofilm is collected at the bottom of the filter. The ideal filter packing material has a high specific area, is inexpensive, and has a high enough porosity to avoid clogging. As wastewater flows over the attached biofilm, it degrades and is collected in the underdrain system. The influent distribution system has more than 2 arms with hollow nozzles through which wastewater is discharged over the filter bed.

Trickling filter media is covered with a visible film as a result of BOD removal. This is called biofilm and it is where soluble substrates and dissolved oxygen diffuse to support biomass growth. As biofilm thickness increases, the substrate cannot penetrate the inner depths of biofilm and microorganisms lose their ability to attach to the media . The applied wastewater then lifts the biofilm off the packing material. Sloughing is a function of organic and hydraulic loading.   Recirculation of trickling filter to trickling filter inlet is the best way to improve trickling filter efficiency. It reduces the strength of the filter inlet and dilutes toxic waste, maintains a good constant wetting rate, forces sloughing to occur, better control of biofilm thickness and provides more oxygen in influent wastewater flow.[12]

Table 1 - Advantages and disadvantages of trickling filter[13]
Advantages Disadvantages
Less energy required Relatively low BOD removal
Simplicity of operation Poor effluent TSS quality
No bulking sludge problems High sensitivity to lower temperature
Better recovery from shock toxic load

and less maintenance required

Uncontrolled solids sloughing events
Schematic of trickling filter [14]

Anaerobic processes:

The anaerobic processes (anaerobic digestion) involve specialized microorganisms that use different electron acceptors in the absence of molecular oxygen for energy production.These processes are used for anaerobic oxidation of wastewater organics, reduction of nitrate to nitrogen gas, and anaerobic digestion of waste sludge. Anaerobic processes are further divided into hydrolysis, acidogenesis, acetogenesis and methanogenesis.

Different stages of anaerobic processes

Tertiary treatment

The main purpose of advanced or tertiary treatment is to meet stringent discharge limits by removing nitrogen and phosphorus from the wastewater. Both processes are subdivided into chemical and biological processes.

Nitrogen removal

Physical and chemical process

Ammonia in wastewater exists as ammonium hydroxide.

Physical processes involve the addition of lime to increase the pH to 10.8-11.5 which converts ammonium to ammonia gas. Chemical processes are breakpoint chlorination, which is described by following equation.

Biological nitrogen removal

Biological nitrogen removal is categorized into nitrification and denitrification.

Biological nitrogen removal processes
Nitrification process

Oxidation of ammonium (NH4+) to nitrite (NO2-)

Oxidation of ammonium (NH4+) to nitrate (NO3-)

Denitrification process

Denitrifying bacteria can oxidize organic substrate using nitrate/nitrite as an electron acceptor.

Carbon source: organic substrate in the wastewater is calculated using the formula for organic substrate decomposition below:


Carbon source: methanol as the electron donor is calculated using the formula for methane oil below:

Biological nitrogen removal processes are traditional single aerobic tank, post and pre-anoxic denitrification, four-stage BNR (Barden Pho) process, step feed process, oxidation ditch and two-stage segregated process.

Phosphorus removal

Phosphorous is present as organic phosphate, polyphosphate and orthophosphate in wastewater. Chemical and biological removal are the two types of processes utilized.

Chemical process

Phosphorous is chemically removed from wastewater by precipitation.  

Using aluminum sulfate:

The chemical removal method is  an effective and widely used method. It is a simple process and easy to control. The phosphorus removal calculation using aluminum sulphate is shown below:

Biological phosphorus removal

The biological phosphorous removal process involves a Pho strip process, a Pho strip with anoxic/ aerobic ASP process, modified Barden Pho process, Pho redox (A/O) process, Anaerobic/Anoxic/Aerobic (A2/O) Process, and a sequencing batch reactor process.

In Phostrip process, phosphorous is removed in waste biological and chemical sludge. Ortho phosphorous is released in the anaerobic tank and moves into the aerobic tank. The supernatant of the anaerobic tank (containing Ortho phosphorous) is taken to the lime reactor. Lime is added to the precipitator tank raising the pH, which causes precipitation of . A large percentage of phosphorous is removed as lime sludge in chemical phosphorous removal process than biological phosphorus removal process. The anaerobic tank is a thickener which creates an interface between the sludge bed and the supernatant. The released phosphorous in the sludge is to be transferred to the supernatant (Elutriation).  Elutriation is accomplished by recycling the thickener underflow to the inlet or by passing an elutriation stream through the sludge bed. In inlet of influent stream, PAOs (Polyphosphate-accumulating organisms) take up phosphorous under anoxic and aerobic condition. The elutriation stream can be primary effluent, secondary effluent, or supernatant from the lime precipitation reactor. [15]

Phostrip Process

Sludge removal

Sludge treatment is required for volume reduction of sludge to ease and reduce handling cost, create stabilization to reduce odor and health risk problems, and sludge thickening purposes.

Gravity thickeners are the most utilized where sludge is fed from the center of the basin. Thickened sludge is removed from the bottom. The pickets stir the sludge blanket very slowly to help release water. The clarified supernatant is returned to the primary sedimentation tank. The average retention time is 24 hours and solid removal is around 95%. In this method, Primary sludge can be thickened up to 8%, secondary sludge can be thickened up to 3%, and mixed sludge up to 6%.

References

  1. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 20, 20–2. Michigan State University: Mc Graw Hill.
  2. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 20, 20–46. Michigan State University: Mc Graw Hill.
  3. GLUMRB (2004), Recommended Standards for Wastewater Facilities, Albany, New York: Great Lakes–Upper Mississippi River Board of State and Provincial Public Health and Environmental Managers, Health Education Services, pp. 60-1-60-9.
  4. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 20, 20–08. Michigan State University: Mc Graw Hill.
  5. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 20, 20–8. Michigan State University: Mc Graw Hill.
  6. https://commons.wikimedia.org/wiki/File:Dibujo_MID1.1.PNG
  7. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 21, 21–2. Michigan State University: Mc Graw Hill.
  8. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 21, 21–4. Michigan State University: Mc Graw Hill.
  9. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 21, 21–3. Michigan State University: Mc Graw Hill.
  10. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 21, 21–3. Michigan State University: Mc Graw Hill.
  11. Mackenzie L. Davis 2010. Water and wastewater engineering, Design Principles and Practice , Chap. 23, 23–2 Michigan State University: Mc Graw Hill.
  12. Metcalf et al. 2014. Wastewater Engineering, Treatment and Resource Recovery, fifth edition, Chap. 9, 9–2. New York City: McGraw-Hill Education.
  13. Metcalf et al. 2014. Wastewater Engineering, Treatment and Resource Recovery, fifth edition, Chap. 9, 9–2 (pp. 953-954). New York City: McGraw-Hill Education.
  14. https://commons.wikimedia.org/w/index.php?search=Trickling+filter&title=Special:MediaSearch&go=Go&type=image&haslicense=unrestricted
  15. Metcalf et al. 2014. Wastewater Engineering, Treatment and Resource Recovery, fifth edition, Chap. 8, 8-8 (pp. 870). New York City: McGraw-Hill Education.