Wastewater Treatment

• Definition of wastewater

  • Water affected in its quality and becomes liquid waste

  • Domestic sewage, industrial effluents, storm water and infiltration water

  • Main pollutants: suspended solids, biodegradable organic matter, nutrients, heavy metals and pathegenic organisms

• Objetives of treatment

  • To reduce water pollution and consequent damage to aquatic ecosystems

  • To avoid harmful effects of pollutants on human health

  • -> to produce an effluent suitable for re-use or for environmentally-safe disposal

• definition od maximum acceptance during wet weather conditions (QM)

• Depended on size and net sturcture

  • rural catchments 6 to 9

  • highly urbanized catchments 3 to 6

• Typical Loads

• Typical Concentrations

• Removal of disturbing substances (e.g. Sand), settleable or floating matter

• Screens

  • seperation of coarse solid particles

  • Protection of pumps

  • Difficulties with hair, textile fibers, wet wipes and plastics

  • Seperation efficiency depends on

    • width of opening (mm)

    • Inflow turbulences

    • Head loss within the system

• Grift Chamber

  • Largely removal (ca.95%) of mineral substances sand/ grit > 0,1-0,2 mm, i.e. v>10 mm/s

  • Seperation of mineral substances from putrescible organic substances

  • Machine protection in subsequent processes from sanding and abrasion

  • Crucial sizing parameters

    • Surface loading rate

    • Flow velocity

    • Retention time HRT

• Aerated Grit Chamber

  • Construction and Operation Principles

    • Long, shallow, aerated settling tank

    • Aeration tubes located deep down in the side of the chamber

    • Rising air bubbles create an upward water flow

    • Longituginal spiral flow along the channel

    • Sand/ grit slides into the collection channel

    • Greas float towards fat trap

• Primary Settling tank

  • Removal of suspended organic solids

  • Protection against clogging of downstream units e.g.

    • Trickling filter

    • Fine pore aeration systems

    • Filter nozzles

  • Reduction of oxygen consumption and quantity of excess activated sludge of the biological treatment

  • Pre-thickening of primary sludge

  • Energy recovery from carbon-rich primary sludge with anaerobic digestion

  • Rectangular settling tank with chain scrapper

• Pathways of C,N and P

• Fundamentals of Biological Treatment

• Aeration tank and Secondary settling tank - dependent units

• Sludge loading -> “food-supply”

  • High sludge loading

    • high food supply

    • Many storage compounds

    • High biomass production

    • Lower removal efficiency

  • Low sludge loading

    • Low food supply

    • Few storage compounds

    • Lower biomass production

    • High removal efficiency

• Remember the unit processes: growth rate of heterotrophs and autotrophs - the process of nitrification only starts when there is a growth limitation for the heterotrophs

• BOD5 Flows in WWTP

Fundamentals of the process (simplified) - MO = microorganisms:

• Aerobic heterotrophic carbon removal

• Aerobic chemo-autotrophic nitrification

• Anoxic heterotrophic process denitrification

• Biological process options

  • pre-anoxic denitrification

    • defined reactor volumes

    • good to control

    • Suitable for big wastewater treatment plants

  • Simultaneous/ intermittent nitrification - denitrification

    • large reactor volumes

    • no recirculation for denitrification

    • Suitable for small WWTP

  • Post-anoxic denitrification

    • external carbon source needed

    • No recirculation for denitrification

    • More suitable for particular cases

• Process steps of precipitation and coagulation/ flocculation

• Phosphorus Flow in WWTP

• Purpose of a secondary settling tank

  • Seperation of the activated sludge from the treated water phase

  • Maximum retentiion of solids and retun the sludge into the aeration tank

  • Responsible for keeping the surveillance values (as last treatment step)

• Therefore the following tasks have to be fulfilled

  • Separation of sludge from water phase

  • Thickening and scraping of the sludge in order to return it into the biological reactor

  • Storage of sludge

  • Avoidance of flows which lead to sludge losses

• Type of secondary settling tanks