Wastewater Treatment: Processes, Technologies, and Management

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Wastewater Treatment

Sludge Density Index (SDI)

The weight in grams of one mL of sludge after settling for 30 minutes. SDI = 100/SVI.

Process Design Examples

  • Complete Mix Activated Sludge (CMAS): Air is blown into the tank to keep the waste mixed with the organisms and to add oxygen to the water. This process removes soluble suspended solids (SS) and ammonia by exposing the waste to organisms.
  • Sequencing Batch Reactor (SBR): This reactor operates under non-steady state conditions in a batch mode with aeration and sludge settlement occurring in the same tank. The SBR tank carries out the functions of equalization, aeration, and sedimentation in a time sequence.
  • Staged Nitrification Process: Offers greater treatment efficiency and capacity than a single complete mix reactor.

Biological Nitrogen Removal (BNR)

All BNR processes include an aerobic zone where biological nitrification occurs. Some anoxic volumes and times are required for denitrification and total nitrogen removal.

  • Examples of BNR include “anoxic/aerobic process,” “step-feed anoxic/aerobic process,” “intermittent aeration,” “sequencing batch reactor,” and “post-anoxic denitrification with methanol addition.”
  • Key design factors for the anoxic/aerobic process include:
    • Anoxic detention time
    • Mixed liquor volatile suspended solids (MLVSS) concentrations
    • Internal recycle rate and return sludge flow
    • Influent BOD or biodegradable COD (bCOD) concentration
    • Readily biodegradable COD (rbCOD) fraction
    • Temperature

Biological Phosphorus Removal (BPR)

Differences Between A/O, A2O, and UCT:

  • Phoredox (A/O) Process:
    • No nitrification
    • Low operating SRTs to prevent nitrification (i.e., SRT: 2-3d at 20°C; 4-5d at 10°C for BPR to occur without nitrification)
    • For nitrification, the denitrification process must be involved with RAS (return activated sludge)
  • A2O and UCT:
    • Nitrate removal with BPR
    • UCT is more widely used for weak wastewater where the addition of nitrate causes significant effects on the BPR performance

Biological Treatment with Membrane Separation

  • Membrane Biological Reactor (MBR) Systems: Aerobic or anaerobic suspended growth bioreactors separating treated wastewater from the active biomass.

Advantages and Disadvantages of the MBR Process

Higher loading rateHigh capital cost
Shorter reactor hydraulic retention time (HRT)High energy cost
Longer SRTs = less sludge productionLimited data = membrane life
Even at low DO, nitrification and denitrification occurPotential high cost: periodic membrane replacement
High-quality effluentMembrane fouling
Less space required

Sludge Disposal and Treatment

Primary Goals:

  • Stabilization of the organic matter contained in the sludge
  • Reduction in the volume of sludge for disposal by removing some of the water
  • Destruction of pathogenic organisms
  • Collection of by-products (possibly used or sold to offset some of the costs of sludge treatment)
  • Disposal of the sludge in a safe and aesthetically acceptable manner
  • Treat coarse primary solids and secondary biosolids accumulated in the wastewater treatment process, as they may contain toxic organic and inorganic compounds

Methods for Residual Treatment:

  • Thickening: Increases the solid contents of sludge by removing liquid fractions.
  • Stabilization: Reduces sludge’s volatile contents.
  • Sludge Conditioning: The addition of chemicals to influent sludge to improve dewatering characteristics.
  • Dewatering: Reduces the water content of sludge.
  • Sludge Disposal: The most common options are land application and landfilling.


  • Thickening is normally achieved by gravity settling, flotation (DAF), centrifugation (by inducing centrifugal forces), or gravity belts.
  • Sludge thickening can significantly affect the cost and size of downstream sludge treatment processes, such as dewatering, digestion, and sludge hauling.


  • Wastewater sludge poses health hazards (biological sludge contains pathogens that are problematic if not properly managed, in addition to odor issues.
  • Purpose: To reduce pathogen contents and reduce the potential for decomposition.
  • Methods: Reduction of sludge volatile content by:
    1. Chemical oxidation
    2. Chemical addition to provide environmental conditions in which microbes cannot survive
    3. Disinfection

Examples of Stabilization

Lime StabilizationAnaerobic Digestion (Most Commonly Used)Aerobic DigestionComposting
  • Inactivation of microorganisms by increasing pH – hydrated lime (Ca(OH)2) or adding quicklime (CaO) before dewatering or after dewatering to raise pH above 12.
  • Advantages: Reliability, low capital costs, space efficiency, ease of operation.
  • Disadvantages: No reduction in solid mass, which actually increases due to the addition of lime.
  • Used to stabilize solids, reduce volatile solids, and remove pathogens.
  • Beneficial by-product of anaerobic biotransformation: Methane gas (used as an energy source).
  • Process factors (solids retention time, hydraulic retention time, pH, nutrient availability, temperature, mixing, substrate, and bioavailability) need careful analysis.
  • If one of these factors is neglected, process efficiency may suffer.
  • Oxygen is supplied to microorganisms.
  • Advantages:
    • Volatile solids reduction meets or exceeds that of anaerobic digestion.
    • Stabilized sludge is free of offensive odor and is an excellent fertilizer.
    • Lower supernatant BOD concentrations than those of anaerobic digestion.
  • Disadvantages:
    • Relatively easy operation.
    • Cost of oxygen supply to microorganisms.
    • Power cost for supplying oxygen.
    • Poor dewatering characteristics.
    • Process performance depends strongly on temperature.
    • More commonly used at smaller municipal wastewater treatment plants.
  • Aerobic stabilization process is used.
  • Sludge is converted into humus-like material.
  • Exothermic process.
  • Composted biosolids are used in landscaping applications, topsoil blending, gardening, and requiring a supplementary nutrient addition if used as fertilizer.
  • Process: Mixing sludge with dry porous media, allowing for air circulation through the mixture.
  • Major design parameters: Selection of bulking agents, mixture ratio, moisture content, aeration requirements, temperature, C:N ratio.

Sludge Conditioning

  • Prior to dewatering, chemicals are often added to influent sludge to improve its dewatering characteristics.
  • The addition of chemicals increases solids concentration, but the physical properties of sludge are altered to aid in dewatering efficiency.
  • Common chemicals are lime, alum, ferric chloride, and organic polymers.


  • Reduces water content of sludge.
  • Advantages:
    1. Easy to handle
    2. Reduced hauling cost
    3. Possibly required for subsequent sludge treatment including incineration and composting
  • Common methods:
    1. Centrifugation
    2. Belt filter press
    3. Pressure filter press
    4. Sludge drying beds

Sludge Disposal

  • Land application and landfilling
  • Partial disposal: incineration, pyrolysis, and wet air oxidation

Supplementary Information

Water Pollution Control Overview

  • Legislature: This limits the extent of contamination back to natural freshwater sources (i.e., Clean Water Act and Great Lakes Legacy Act).
  • Water Recycling: Taking wastewater and re-treating it for non-potable water uses (i.e., field irrigation).
  • Water Conservation: By practicing water conservation, there is less wastewater that runs off into natural habitats.
  • Responsible Consuming: Being a responsible consumer means also being a major participant in the fight against water pollution.
  • Treatment: Physical, chemical, biological, or combined (integrated) treatment options.

Best Available Technology

Selection of Technology

  • Technology Selection Process: Multi-criteria optimization considering technological, logistic, environmental, financial, and institutional factors within a planning horizon of 10–20 years.
  • Key factors include:
    • Size of community to be served
    • Characteristics of sewer system
    • Sources of wastewater (i.e., domestic, industrial, stormwater, infiltration)
    • Future opportunities to minimize pollution loads
    • Discharge standards for treated effluents
    • Availability of local skills for design, construction, and O&M
    • Environmental conditions (i.e., land availability, geography, climate)

Onsite Technologies

  • For domestic wastewater, the suitability of various sanitation technologies must relate to the type of community (i.e., rural, small town, or urban).
  • Typically in rural areas, onsite sanitation systems are most appropriate because of low cost due to the absence of sewerage requirements, easy construction, repair and operation by the local community or plot owner, and effectiveness.

Offsite Technologies

  • Primary Treatment: Physicochemical processes may be incorporated.
  • Secondary Treatment: Biological treatment (most common).
  • Tertiary Treatment: Reduction of recalcitrant contaminants. Examples of tertiary treatment technologies include membrane filtration and separation, dechlorination and disinfection systems, reverse osmosis (RO) systems, ion exchange, activated carbon adsorption, integrated physical and chemical treatment.


  1. Direct Re-use: Some slightly soiled waters may be used again for some purpose not requiring high quality. No national regulation for water reuse; no regulation for the application for reclaimed water as a drinking water supply.
  2. Reuse After Treatment: Some relatively easily treatable water may be kept separate from the general waste stream and treated to bring it up to a standard suitable for some purpose within the factory.
  3. Recycling: This is used repeatedly for the same purpose until the quality is degraded beyond acceptable limits. Benefits: Decreased discharge to sensitive water bodies; enhancing wetlands and riparian habitats; reducing and preventing pollution; and saving energy.
  • Direct reuse and recycling require only a minimum of capital outlay and extremely low running costs.
  • Water Re-use: This is an obvious way to reduce the amount of water that must be extracted from surface water sources.

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