Biological wastewater treatment processes Flashcards
What is the first stage of waste water treatment and what is its purpose
Screening. Removes material that can block pumps
Why is grit removal important
The grit can build up at the bottom of purifying tanks which will reduce capacity
What are the 4 parameters that need to be removed/ heavily reduced in waste water treatment
Suspended solids, BOD (biological oxygen demand), faecal coliform bacteria and ammonia
Give the four main reasons for treating water
health (reducing pathogenic hazards in water), ecology (water must be safe for environment), aesthetic (keep water clean for recreational activities) , economics (provide appropriate treatment at the right cost)
What is primary sedimentation
Removal of settleable and suspended solids which are heavier than water through the use of gravity
What comes after primary sedimentation
Secondary treatment sometimes called biological waste water treatment
what does Secondary sedimentation produce
secondary sludge and the treated effluent
what happens to the secondary sludge
It undergoes sludge treatment such as anaerobic digestion
What are the two main biological treatment processes
attached growth process and suspended growth process
What is the attached growth process
Involves a tank filled with aggregates acting as a filter that have biomass attached to the gravel. The sewage is distributed onto the aggregates then the water movement causes the sewage to flow through the aggregates.
Why do attached growth filters contain an underdrain
located under the aggregates, it is used to collect the treated effluent as well as excess biomass
How does the attached growth process treat the water
The biomass on the aggregates consists of microorganisms which produce enzymes that degrade organic material in the waste water. Organic material and O2 diffuse into the biofilm where it is consumed by the biomass and the biomass thickens, eventually becoming detached from the aggregate. Could also contain nitrifying bacteria to convert ammonia into nitrates
What are the advantages of attached growth processes
low energy use, low maintenance, easy to operate, can withstand shock loads (toxic chemicals etc), biomass is attached so is retained.
What are the disadvantages of attached growth processes
stinky, lower level of control (hard to remove nutrients and phosphorous) , fly nuisance, filter clogging, more difficult to operate in cold weather
What is the suspended growth process
The microorganisms are suspended in the mixed liquor as agglomeration of cells in an extracellular gelatinous matrix called a FLOC. These are contained within an aerated reactor and are continuously removed from the tank (as it flows with the water) and then filtered by the secondary clarifier then some are recycled back into the aerated tank.
