5. Wastewater treatment I

Composition of wastewater:

Domestic wastewater contains human and animal faeces and urine plus gray water from washing, cooking, bathing etc.

Objectives of wastewater treatment processes

The aim of wastewater treatment is to reduce organic and inorganic materials in wastewater to a level that no longer supports microbial growth, and to eliminate other potentially toxic materials. Specifically:

  1. Reduction of organic content of wastewater

  2. Removal/reduction of trace organics recalcitrant to biodegradation that may be toxic

  3. Removal/reduction of toxic metals

  4. Removal/reduction of nutrients to reduce pollution of receiving waters

  5. Removal/inactivation of pathogens

Overall Aims:

  • Wastewater must first be characterised

  • The treatment process must be monitored

  • The treated wastewater and sludge must be safely disposed of

Monitoring:

What needs to be monitored:

Organic carbon

  • requires oxidation, measured as BOD or COD

Inorganics

  • nutrients such as N, P, S

  • toxic metals, mainly from industrial activity, mining

Special organics

  • volatiles, petroleum hydrocarbons

  • non-volatile, polychlorinated biphenyls (PCBs), pesticides

Microbiology

  • pathogens present in human waste, not routine to monitor these

  • indicator organisms to monitor faecal pollution

What needs to be achieved?

Typical composition of untreated domestic wastewater and some effluent guidelines.

Routine Analyses of Organics in Wastewater:

  1. BOD - Biochemical Oxygen Demand

  2. NOD - Nitrogenous Oxygen Demand

  3. COD - Chemical Oxygen Demand

  4. Total Organic Carbon

  5. Suspended Solids (SS)

(1) BOD - Biochemical oxygen demand → the amount of dissolved oxygen (DO) consumed by microorganisms for the biochemical oxidation of organic (carbonaceous BOD or CBOD) and inorganic (nitrogenous BOD or NOD) matter

CBOD5 or BOD5 - a 5-day measurement of the amount of O2 required to oxidise organic matter in a sample:

BOD is one of the most important tests in monitoring of wastewater treatment plants

BOD5 measurement

Aliquots of wastewater placed in bottle, diluted, may be seeded with culture, sample is saturated with oxygen, left in dark

DO is measured at start and after 5 days incubation at 20oC. Amount of O2 consumed is determined.

As microbes deplete the carbon substrate a plateau is reached.

(2) Nitrogenous Oxygen Demand (NOD) → amount of DO required for biological oxidation of organic nitrogen in wastewater

  • is due to nitrifying bacteria that oxidise NH4 + to NO3 -

  • usually occurs after the BOD5 due to slower growth of nitrifiers

  • addition of nitrification inhibitor enables distinction between carbonaceous BOD (CBOD) and NOD

(3) Chemical Oxygen Demand (COD) → amount of required O2 to oxidise organic carbon completely to CO2, H2O & NH4 + (independent of microbes)

organic compounds are oxidised to CO2 by adding a strong oxidising agent (e.g. potassium dichromate) under acid conditions

the BOD5/COD ratio is the fraction of easily degraded organic matter by microorganisms

(4) Total Organic Carbon → Represents the total organic carbon in a given sample independent of oxygen state

determined by oxidation of heated organic matter & measurement of CO2 liberated

(5) Suspended Solids (SS) → Measured by filtering sample, drying the residue (104˚C), and determining the mass. SS released in the effluent can foul waterways

Mixed liquor suspended solids (MLSS) A measure of the suspended solids in an aeration tank during the activated sludge process. It also indicates the concentration of biomass in a wastewater treatment plant (WWTP)

Wastewater treatment processes

  1. Primary and preliminary treatment

    Objective is to remove solids, debris and coarse materials Reduced BOD: by 30-40%

  2. Secondary treatment

    Biological and physical processes. Objective is to speed up the biological degradation

    Reduced BOD: by 80-90%

  3. Tertiary treatment

    Secondary effluent may be treated further to obtain a higher effluent quality

The treated wastewater and sludge must be disposed of safely (sludge is the semi-solid material left after the wastewater treatment process)

1. Primary and preliminary treatment:

uses physical processes such as screening and sedimentation

2. Secondary treatment:

Different types of secondary treatment

1)Trickling filters

2)Rotating Biological contactors

3)Activated sludge treatment

4)Anaerobic digestion (also a type of sludge stabilisation)

Waste stabilisation ponds (lagoons)

Soil aquifer treatment

Artificial and constructed wetlands

Secondary Treatment: 1. Trickling Filter

  • A type of Aerobic Attached-Growth Process (developed 1900s)

  • Watertight basins filled with rocks or highly permeable plastic medium

  • Wastewater is trickled through medium and air diffuses up from bottom

  • Microbes attach to the medium to form a biofilm and degrade the organic matter in incoming water to reduce BOD

  • Second most common type of WWT

Secondary Treatment 2. Rotating Biological Contactors (RBC)

  • A type of Aerobic Attached-Growth Process

  • Circular disks (PVC) that are submerged in the wastewater and rotated slowly through it

  • Biological growth forms a slime layer on discs; rotation maintains biomass in an aerobic condition

Secondary Treatment 3. Activated sludge wastewater treatment

  • A type of Aerobic Suspended Growth Processes (suspended biofilm)

  • The most common form of wastewater treatment (aerobic tank shown below)

Degradation of organic matter in activated sludge

Settling

Required for removal of solids from treated wastewater following the aerobic suspended growth processes

  • Sewage is mixed with a mixture of microorganisms called activated sludge.

  • The sewage and sludge are aerated to allow the microorganisms to break down organic matter.

  • This process, known as biological oxidation, converts pollutants into carbon dioxide, water, and more microorganisms.

  • The mixture then goes to a settling tank where the activated sludge settles to the bottom, forming sludge.

  • The treated water is separated from the sludge and discharged or further treated.

  • The sludge can be recycled back into the process or undergo further treatment.

Sludge processing

  • Sludge treatment and disposal is costly

  • Sludge from primary and secondary processes contains ~ 1-5% solids

  • Treatment of sludge involves thickening and dewatering by centrifugation and filtration

  • Results in sludge with a solids content of 20-40%

  • Dewatering is followed by stabilisation and final disposal of sludge

Sludge processing – Sludge stabilisation

The purpose of stabilisation is to break down the organic fraction of the sludge to reduce its mass, reduce odour and make it safer

Sludge is first dewatered, then processed by:

  1. Anaerobic digestion

  2. Composting

  3. Sludge may be heat stabilised or lime stabilised and used as fertiliser

  4. Incineration

  5. Land fill

Sludge processing – Sludge stabilisation – Anaerobic Digestion (AD)

2.4 Anaerobic (anoxic) Sludge Digestion

Used as a form of secondary treatment or sludge stabilisation

A series of microbiological processes that convert organic compounds to CH4 and CO2 and reduce volatile solids by 35-60%

Carried out in the absence of O2 by Bacteria and Archaea

Uses CO2 as electron acceptor (anaerobic respiration)

  • One-stage digestion uses single tank; sludge digestion and settling occur at the same time

  • Two-stage digestion process uses one tank for heating and mixing and a second for thickening and storage

  • CH4 produced can be used to generate electricity

Acidogenic bacteria carry out the initial stage: production of organic acids. Methanogenic bacteria convert the acids or CO2 and H2 to methane. These are are forms of Anaerobic Respiration

Sludge processing – Aerobic Sludge stabilisation – Composting

(a) Aerated static pile sludge is mixed with e.g. woodchips and is aerated for 20-30 days, then cured for a further 30 days dried and screened

(b) Windrow - pile is turned and mixed during composting Lasts 30-60 days

3.Tertiary treatment

Disinfection →

Chlorination and UV treatment most commonly used to disinfect secondary effluent (ozone is also used) where microbial contamination persists

(a) UV Disinfection

  • Wastewater is irradiated with UV light

  • Microbial inactivation is by DNA damage

  • UV treatment is an effective (though expensive) disinfectant

(b) Chlorination

  • Chlorine gas (Cl2) or chlorine dioxide (ClO2) is introduced to water to form hypochlorous acid

Cl2 + H2O ←→ HOCl + H+ + Cl-

Chlorine contact chambers are maze-like design to provide maximum contact time with the water

Pathogenic microorganisms in wastewater

  • Bacteria are the most numerous pathogens in wastewater; most common are Salmonella, Vibrio cholerae, Shigella; generally require a high dose to cause illness

  • Most frequent waterborne illnesses are caused by enteric viruses; shed in high numbers in faeces; not removed by filtration e.g. norovirus, rotavirus

  • Protozoa produce cysts and oocysts that can withstand harsh environments; are chlorine resistant e.g. Giardia, Cryptosporidium (most common waterborne infection in UK)

  • Helminths pose serious problems, particularly in developing nations e.g. Ascaris lumbricoides

Tertiary treatment – Other treatments

Suspended Solids Removal

Often required to remove residual biological flocs from secondary effluent Wastewater is filtered through a porous medium (sand); solids are trapped in the medium

Precipitation of inorganics

Often carried out during secondary treatment

Chemical precipitates can are used to remove various inorganics e.g. phosphate

Al3+, Fe2+, Fe3+, Ca2+ form insoluble salts: AlPO4, FePO4 or Ca5(PO4)3OH

  • Soluble Al, Fe, Ca salts are added to the aeration tank in activated sludge

  • The precipitated salts are separated from the wastewater in the secondary clarifier and removed with the sludge

  • Salt dose is in the range of 1-3 metal : 1 P (molar ratio) and can lower effluent P levels to 0.5 mg/L

Wastewater Treatment Lecture 1: Summary

  • Overview of a typical wastewater treatment plant

  • Wastewater treatment requires removal of

    • Organic carbon, N and P, various contaminants, pathogens

  • Organics monitored by measuring

    • BOD, NOD, COD, total organic C, suspended solids, MLSS

  • Levels of treatment include

    • Preliminary and primary, secondary and tertiary

  • Secondary treatment

    • Aerobic growth by

      • (a) Attached biofilm or

      • (b) Suspended biofilm → activated sludge the workhorse of WWT

    • Sludge stabilisation

      • (a) Anaerobic digestion → series of digestive and fermentative reactions carried out by various microbes in anaerobic sludge digester – now also used as a type of secondary treatment or

      • (b) Composting → static pile or windrow

  • Tertiary treatment

    • Disinfection → e.g. chlorine, UV

    • Removal of suspended solids → filtration

    • Removal of inorganics → salt precipitation (may be done during secondary treatment