Lesson Four
Use of Aquatic
Environment sDigestion in Aquatic Environments
In any stationary body of water two layers develop with the oxygen that enters the water from the surface being used up in the upper layer by aerobic and facultative bacteria. The aerobic bacteria produce Co2 and non-smelly products when they breakdown the organic mater.
In the bottom layer all the oxygen is absent so anaerobic bacteria predominate. Theses anaerobes
Digest organic mater to produce the noxious gases H2S, NH3 and CH4. The bad smelling gases don’t escape to the environment because they are oxidised to H20, NO2-, SO4 and CO2 as they rise into the aerobic layer.
Sewage Treatment Lagoons
Sewage treatment plants can utilise the aquatic environments by constructing lagoons. Lagoons have the advantage over land and grass filtration in that they can process a wide rage of through puts. Land and
Grass filtration can be saturated if too much water is applied. Lagoons can vary the though put by over 10 times the volume This is because the more material put through the lagoons the faster the bacteria grow just like a fermentation Chemostat
Chemostat
Sewage Treatment Lagoons
Lagoons were first constructed with up to twelve ponds in series with the deepest pond receiving the raw sewage. This was deep so that it could accommodate a large amount of anaerobic digestion. As treated sewage is drained from one lake to the next the water become more and more purified with less BOD. The water is visible fresher. The first lake has more surface tension and appears Flat and still and are devoid of bird life. The later ponds develop ripples and waves on their surface and the increased oxygen content allows plankton to grow and attract birds.
Ecosystem
A divers ecosystem develops and hundreds of bird species make the lagoons home or visit them on their migration.
Authorities have started harvesting the zooplankton for sale as fish food.
Serpentine Lagoons at Werribee
Newer Lakes are built as long winding lakes, Deep at the inflow end and shallow at the out put end. The serpentine shape slows the flow so that the lagoon is equivalent to the multiple pond system but take up less land and is easier to operate
Read about the lagoons at Werribee on the Melbourne Water web page:
http://www.melbournewater.com.au/mainFrameset.asp?path=/environment/environment.asp
Capture of Methane
To reduce the smell and use the methane produce by the anaerobic region of the serpentine lakes Melbourne Water covers the input regions with large plastic sheets and collects the gases. These gases are used to power engines that generate electricity.
Marine Disposal
A marine environment will process sewage in a similar way as the lagoons. If the affluent is discharges far enough out in the ocean the entire BOD will be removed before any reaches the coastal region. Such affluent must have had primary treated to remove objects that could wash up on shore. The massive amount of non-salty water will alter the ecosystem in the area of discharge causing a different range of organisms to develop.
Reed Beds can also be used to treat wastewater and is a cross between lagoons and grass filtration
Read about the reed bed system provided by Johnstone Smith Co. at http://www.johnstonsmith.co.uk/fact17.html
Constructed Wetlands
Read about the use of constructed wert lands to dispose of effluent with out the need to drain it into the sea or rivers. The water is lost by evaporation.
Read the information in the Constructed Wet lands Web Page then return here by clicking your "Back Button"