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Wastewater Treatment Frequently Asked Questions



What are the sources of wastewater?

There are mainly two (2) wastewater sources: 1) domestic sewage, and 2) non-sewage. Domestic sewage includes all wastewater generated by residential communities, public restrooms, hotels, golf courses, restaurants, schools, hospitals and other health centers. Non-sewage wastewater entails industrial wastewater, stormwater, runoff, water from swimming pools, car garages, and cleaning centers. 

What wastewater is comprised of?

 The composition of wastewater is a function of its source. Domestic sewage/municipal wastewater is typically comprised of pathogenic microorganisms, biodegradable organics, nutrients (such as nitrogen and phosphorous), suspended solids, and may contain toxic compounds that may be carcinogenic. Industrial wastewater usually contains higher organic concentrations (e.g., BOD, COD, TOC, etc), heavy metals, and suspended solids. 

What drives wastewater treatment?

Human health protection

  • Pathogens
  • Chemicals

Environmental Protection

  • BOD and COD
  • Nutrients (e.g., N and P)
  • Metals
  • Endocrine disrupting compounds (EDCs), pharmaceuticals, and personal care products (PPCP)

Recover useful products

  • Water (grey water, aquifer recharge, potable water)
  • Treatment chemicals (e.g., lime)
  • Energy (e.g., methane and hydrogen)
  • Fertilizer

What is biological wastewater treatment?

The removal of dissolved and particulate biological oxygen demand (BOD) and the stabilization of organic matter present in water can be achieved biologically using variety of microorganisms. They are responsible to oxidize the dissolved and particulate carbonaceous organic matter to mineral constituents of CO2 and H2O, and additional biomass.

v1 (organic material) + v2O2 + v3NH3 + v4PO43- → v5 (new cells) + v6CO2 + v7H2O

 Where vi= the stochiometric coefficient

What are the typical biological processes for wastewater treatment?

  • Suspended-growth processes
  • Attached-growth processes
  • Combined processes
  • Lagoon processes

What does biological oxygen demand (BOD) and chemical oxygen demand (COD) refer to?

  • BOD: biological oxygen demand (BOD) is the amount of oxygen required by aerobic biological microorganisms to decompose organic material in a given water at certain temperature over a time period.
  • COD: chemical oxygen demand (COD) is a measurement of the oxygen required to oxidize soluble and particulate organic matter in water using a strong oxidizing agent.

Why COD values are greater than BOD values?

COD values are always greater than BOD values because COD captures both biodegradable and nonbiodegradable substances, whereas BOD only entails biodegradable ones. 

What does nitrification and denitrification refer to?

  • Nitrification: The two-step biological process by which ammonium (NH4+) is converted first to nitrite (NO2-) and then to nitrate (NO3-).
  • Denitrification: the biological process by which ammonia (NO3-) is converted to nitrogen (N2) and other gaseous end products. 

Where does all the sewage go?

All the sewage is collected through wastewater collection systems (combined or separate sewer), from which it is transferred to wastewater treatment plants (WWTP). 

What is the difference between suspended-growth and attached growth processes?

In suspended-growth processes microorganisms are maintained in suspension within the liquid, whereas is attached-growth processes (i.e., fixed-film processes) microorganisms are attached to some inert medium, such as rocks, slag, or specific ceramic or plastic materials. 

What does substrate refer to in biological treatment context?

Substrate denotes the organic matter (e.g., carbonaceous organic matter) or nutrient that are converted during bio-treatment or that may be limiting biological treatment. 

 What is solid retention time (SRT)?

SRT is the average time the activated sludge solids are in the system. The SRT is an important design and operating parameter for the activated sludge processes. 

How can SRT be controlled in an MBR system?

In an MBR system, no solids can pass through the membrane and hence the SRT is defined only by wasted solids. The SRT can be controlled by periodically discharge some of the solids (sludge) from the process. 

What are the advantages of MBR against conventional activated sludge processes (CASP)?

 As a wastewater treatment technology, MBR is considered to be a superior technology compared to conventional activated sludge processes (CASP) in which a membrane replaces secondary settling tank of the conventional ASP to separate effluent from activated sludge. The main advantages of MBR technology compared to conventional activated sludge process are: 1) smaller aeration tank and footprint due to the higher mixed liquor suspended solids (MLSS) concentrations and also higher volumetric loading rate, 2) lower sludge production, and 3) improved effluent quality owing to membrane filtration.

How can one determine whether or not membranes are fouled?

Fouling propensity of the membranes is evaluated through monitoring pressure drop across the membranes, which is referred to as transmembrane pressure (TMP), and recovery rate. High TMP values and low recovery rate indicate that membranes are fouled and need to undergo cleaning. 

What is the main advantage of submerged MBRs over side-stream ones?

Submerged MBRs are generally less energy intensive than side-stream MBRs, as employing membrane modules in a pumped side-stream crossflow demand high energy consumption due to the high pressure and volumetric flows imposed. 

What does preliminary treatment refer to?

 Removal of wastewater constituents such as rags, sticks, floatables, grit, and grease that may cause maintenance or operational problems with the treatment operations, processes, and ancillary systems. 

What does primary treatment refer to?

Removal of a portion of the suspended solids and organic matter from the wastewater.

What does advanced primary treatment refer to?

Enhanced removal of suspended solids and organic matter from the wastewater, typically achieved by chemical addition and/or filtration. 

What does secondary treatment refer to?

Removal o biodegradable organic matter (in solution or suspension) and suspended solids. Disinfection is also typically included in the definition of conventional secondary treatment.

What does secondary with nutrient treatment refer to?

Removal of biodegradable organics, suspended solids, and nutrients (nitrogen, phosphorous, or both nitrogen and phosphorous).

What does tertiary treatment refer to?

Removal of residual suspended solids (after secondary treatment) usually by granular media filtration or micro screens. Disinfection is also typically a part of tertiary treatment. nutrient removal is often included in this definition.

What does advanced treatment refer to?

 Removal of dissolved and suspended materials remaining after normal biological treatment when required for various reuse applications. 

What does critical flux concept refer to in MBR context?

IN MBR context, based on the definition, no fouling is observed below critical flux, while above critical flux fouling occurs. 

What kind of pre and post treatment is required for MBR systems?

The selection of pre and/or post-treatment technologies is a function of feed water quality, discharge requirements, and application. Typically, screening is required prior to MBR in order to protect membrane’s surface from any possible damage caused by aggregates of hair or debris. 

What is the purpose of aeration in an MBR system?

There are two (2) main reasons for aeration:

1) To provide dissolved oxygen (DO) for maintaining a viable microorganism population for biological treatment.

2) To keep MLSS in suspension. 

What is stabilization in wastewater treatment context?

 The biological process by which the organic matter in the sludges produced from the primary settling and biological treatment of wastewater is stabilized, usually by conversion to gases and cell tissue. Depending on whether this stabilization is carried out under aerobic or anaerobic conditions, the process is known as aerobic or anaerobic digestion. 

What is a typical sludge treatment method?

Typically, anaerobic digestion (methanogenic treatment) is being implemented for sludge treatment. This can be performed under two different temperature conditions:

  • Mesophilic (about 35°C)
  • Thermophilic (50-60°C)

It is critical to control the pH between 6.5 to 7.5 (for methanogens). The purpose of sludge treatment is to reduce the volume of waste activated sludge (WAS) for disposed. The process can generate both desirable gases (e.g., CH4) and undesirable gases (e.g., H2S). 

What are anoxic processes?

 The process by which the nitrate (NO3-) nitrogen is converted biologically to nitrogen gas in the absence of oxygen (i.e., denitrification).

What are facultative processes?

Biological treatment processes in which the organisms can function in the presence or absence of molecular oxygen.

To learn more, check the most advanced wastewater treatment systems

If you have any questions, consult a wastewater engineer now

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