Dissolved oxygen

Our atmosphere is 20% oxygen (200,000 ppm) whereas pond water is seldom more than 10 ppm (10 mg/l or 0.001%). Below 3mg/l and fish become stressed, and die at less than 2 mg/l * *. 

Oxygen saturation of water can be expressed as a percentage of the greatest amount of oxygen that the water can hold. Surface pressure and temperature both influence saturation levels.

Biological Oxygen Demand

Biological Oxygen Demand (BOD, also called Biochemical Oxygen Demand) refers to the amount of oxygen that would be consumed if all the organic material in one liter of water were oxidized by bacteria and protozoa (ReVelle and ReVelle, 1988) *. Unpolluted natural water has a BOD of 5mg/l or less*. 

Fresh wastewater has high levels of organic matter, thus a high 5 day BOD (e.g. 600 mg/l). Bacteria will break down waste in water while consuming oxygen in the process.  The bacteria count will be high where high levels of readily-decomposable organic material are present in the water. In order for them to degrade that organic material their "oxygen demand" will be high. 

Oxygen dissolves into water mostly by diffusion from the atmosphere into the water surface * *. Where a high BOD is present and oxygen is not being replenished aquatic hypoxia occurs. As dissolved oxygen levels are depleted, the water will become unsuitable for aquatic organisms (e.g. fish), unless the oxygen is replenished. 

Nitrates and phosphates in water bodies can also contribute to high BOD levels because they cause rapid algae and phytoplankton growth and this organic biomass will decay, demanding oxygen in the process * *.

Anaerobic conditions

Once dissolved oxygen in wastewater is depleted, anaerobic bacteria use nitrates present in the watewater to decompose organic matter. This process is known as denitrification and produces nitrogen gas, lowering levels of N in the wastewater. Once all the nitrate is used, bacteria will then reduce sulfate, producing sulphur gas. Both nitrate and sulfate are nutrients required by plants, thus anaerobic decomposition is not desirable if treated effluent is to be a nutrient source for growing plants.

Traditional primary treatment of sewage involves settling of solids from the wastewater in an anaerobic environment such as a septic tank.

Anoxic conditions in sediment reduce the rate of decomposition, thus enrichment of this substrate with organic material occurs. However, if then exposed to an aerobic environment, settled solids can be rapidly decomposed.

Aerobic decomposition

Improving levels of dissolved oxygen in wastewater after primary treatment is usually achieved by aeration. Conventional mechanical aerators include paddles and bubblers, which have the objective of improving diffusion of oxygen at the water surface. This can be energy intensive.

Vermidigesters offer separation of solids from wastewater at source. Removing these solids from the wastewater and aerobically decomposing them means the solids no longer contribute to the oxygen demand in the effluent. 

Vermifilters offer a secondary treatment process for primary treated wastewater. Instead of aerating the wastewater directly, the vermifilter trickles the effluent through a matrix filter of media such as pine bark, wood chips or humus, where the huge surface area of the water travelling through the media combined with aerobic conditions oxygenates the water. Because a biofilm of micro-organisms is attached to the media, the dissolved and suspended solids are also filtered out and decomposed aerobically and fast. To complete the cycle, composting earthworms digest the biofilm and convert this into humus. The humus then becomes part of the filtration substrate. 

Further reading:

• Dissolved oxygen

• Vermifilter (wikipedia)