Unit Processes

Storage = Input - Output + Production - Consumption

• Raw water quality

• Contaminant removal / legal requirements

• Process reliability, flexibility and complexity

• Costs

• Energy needs

• Gas Exchange (Liquid <-> Gas): Aeration, Stripping

• Chemical Reactions (Liquid <-> Liquid): Acid-base and Redox Reactions

• Phase Transition (Dissolved <-> Undissolved): Precipitation, Adsorption

• Liquid-Solid-Seperation (Suspended <-> Liquid): Screening, Sedimentation/ Flotation, Filtration

• refers to solid-liquid seperation in which no chemical changes occur

• Sedimentation: settling of particles through a fluid under the influence of gravity - works best with relatively dense particles

• Flotation: involves (1) generation of air bubbles, (2) contact between the air bubbles and the particles to be removed, (3) flotation of particles by the buoyant force created and (4) removal by skimming

• Filtration: Process of removing solids from a fluid by passing it through a porous medium. The filter media are artificial membranes, granular materials or cartridges

• Key parameter: settling velocity

• Particles should be able to reach the bottom of the tank within the detention time, i.e. overcome the Height H

• v>Q/Asurf=qA

• In a settling tank, all particles having a settling velocity greater or equal than the surface loading will be removed completely

• Removal Efficiency: Particles size: 63 micrometer - efficiency depends on: surface loading rate, sludge loading rate, detention time, flow rate, flow regime, characteristics of the partciles suspension

• Shallow septh sedimentation

• settling area is very large on account of the overlapping of the plates but occupies a relatively small plan area

• Cake Filtration

  • Diatomaceous earth filtration

  • Microfiltration

  • Ultrafiltration

  • Slow sand filtration

• Cross-flow filtration

  • Reverse Osmosis

  • Nanofiltration

• Deep-bed filtration

  • Rapid sand filtration

  • Cartridge filters

• Removal Mechanisms

  • Transport processes by which the partciles are brought into contact with the sand grains

  • Forces that hold the particles attached to the sand grains

  • Contaminants are removed due to chemical and microbiological oxidation (Schmutzdecke on top and biofilm on sand grains)

• Filtration Spectrum

• Process which change the compositiion or physical state of dissolved and suspended solids and involves chemical reactions

• Precipitation: Process whereby a substance in solution is transformed into a solid phase (precipitate)

• Coagulation - Flocculation: Destabilization and initial coalescing of colloidal particles by a adding a coagulant (positively charged). These chemicals ensure the aggregation of the suspended particles and form larger flocs

• Disinfection: To kill or inactivate pathogenic microorganisms using disinfection methods (e.g. Chlorination)

• Neutralization: A base or an acid is added to the fluid for pH control

• Process in which soluble substances are removed from solution by the addition of a reagent with which they react to form a solid (precipitate)

• This precipitate can then be removed by standard flocculation, sedimentation and/ or filtration process

• Phase transition process

• Colloidal particles and very fine suspension are combined into larger agglomerations that can be seperated via phyisical processes

  • Reduce particle repulsion by adding positively charged ions (destabilization)

  • Favors initial coalescing of colloidal particles

• No phase transition process

• Coagulation

  • Most colloidal particles have a negative charge and therefore repeal each other

  • Chemicals are added to neutralize the negative charge on non-settleable solids

    • high energy, rapid-mix to properly disperse coagulant and promote particle collisions (1)

    • Stiring stron enough to cause particle collisions but not to strong as to break up particles

  • One cahrge is neutralized, small suspended particles are capable of sticking togehter - form slightly larger particles called microflocs (coagulation) (2)

• Flocculation

  • Microflocs are brought into contact with each other through the process of slow mixing - collisions cause microflocs to bond and to produce larger pinflocs (3)

  • Organic polymers are added to enhance particle agglomeration

    • Large polymeric molecules have charged sites making flocs attach to the polymers (4)

    • Long polymer chains can interact with other particles forming “bridges”

  • To prevent floc from tearing apart or shearing, mixing velocity and energy are siminished as size of floc increase

• Processes by which organic matter can be biologically degraded if proper microbial communities are established, maintained and controlled

• Process characteristics

  • Environmental conditions

    • Aerobic digestion (Oxygen rich environments; energy yield up to 36 ATP per 1 molecule Glucose)

    • Anaerobic digestion (reguires no oxygen; energy yield 2 ATP per 1 molecule Glucose)

  • Microbial growth

    • Suspended growth processes

    • Attached growth processes (Biofilm and Granular process)

  • Main groups of microorganisms

    • Heterothrofic organisms (Consumer: need organic substance for energy production and growth)

    • Autotrophic organisms (Producer: can use inorganic substance)

• Fixed Film Bio-Reactors

• Aerobic granules (Nereda)

  • Nereda is an aerobic granular technology

• Aerobic organic substrate removal in the presence of oxygen and by heterotrophic organisms

• Heterotrophic organisms use organic material as the carbon source for boysynthesis - a fraction of the carbon source is oxidized to carbon dioxide and water (energy production), the remaining fraction is assimilated for cell growth

• Organic compounds: e.g. Glucose, fats, alcohols (including carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), …)

• Important oxidation reaction that occurs in some wastwater processes

• Ammonia is transformed into nitrites and thes nitrites into nitrates

• Microorganisms involved in these processes are chemoautotrophs:

  • Nitrosomonas - oxidation of ammonia to nitrites

  • Nitrobacter/ Nitrospira: Oxidation of the nitrites to nitrates

• Consumption of free oxygen -> nitrogenous demand

• Release of H+ -> consuming the alkalinity of the medium, reducin pH value

• Nitrifying bacteria (autotrophic microorganisms) have a low growth rate compared to the heterotrophic bacteria

• Factors influencing: pH value (near neutral), Temperature (mesophilic 30-36°C), Substrate availability, dissolved oxygen concentration, sludge age (8 to 10 days), C/N ratio

• Microbial reduction of nitrate to nitrogen gas without the presence of molecular oxygen (anoxic)

• Broad range of bacteria can accomplish denitrification

  • denitrifying bacteria are facultative -> they can use oxygen and nitrate as an oxidant of organic matter

  • As heterothrophic bacteria they need a carbon source: intern (influent organic matter) or extern (methanol)

• Denitrification removes nitrogen from the wastewater by converting it to insoluble gas that escapes to the atmosphere

• Factors influencing: pH (7-8,5), Temperature (max. 40°C), availability of carbon and nitrate, dissolved oxygen concentration

• Aerobic degradation

  • many heterothrophic bacteria

  • e-donator = BOD

  • e-acceptor = oxygen

  • Produciton of biomass: high

  • Sludge age: low

• Anoxic degradation = Denitrification

  • Heterotrophic bacteria capable of denitrification

  • e-donator = BOD

  • e-acceptor = nitrate

  • Production of biomass: suboptimal

  • Sludge age: medium

• Removal of ammonium in two seperate processes:

• First partial nitrification (nitration) of half of the ammonium to nitrite

• Remaining ammonium and nitrite are converted to dinitrogen gas and 15% nitrate by anammox bacteria

• Both processes can take place in one reactor where two groupd of bacteria form compact granules

• Less energy production, less sludge production

• High sludge age: required more than 20 days

• Annamox bacteria are autotrophy

• Break down organic compounds in the absence of oxygen

• Energy-rich biogas is formed from organic matter in several steps

• Essential steps for energy recovery in wastewater treatment plant

• Anaerobic digestion occurs in four steps:

• Biomass growth is much lower compared to those in the aerobic process