Water Reuse

Tertiary Treatment

The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality to the desired level.  This advanced treatment can be accomplished by a variety of methods such as coagulation sedimentation, filtration, reverse osmosis, and extending secondary biological treatment to further stabilize oxygen-demanding substances or remove nutrients.  In various combinations, these processes can achieve any degree of pollution control desired.  As wastewater is purified to higher and higher degrees by such advanced treatment processes, the treated effluent can then be reused for urban, landscape, and agricultural irrigaton, industrial cooling and processing, reacreational uses and water recharge, and even indirect and direct augmentation of drinking water supplies.

Coagulation sedimentation

Chemcal coagulation sedimentation is used to increase the removal of solids from effluent after primary and secondary treatment.  Solids heavier than water settle out of wastewater by gravity.  With the addition of specific chemcials, solids can become heavier than water and will settle.  Alum, lime, or iron salts are chemicals added to the watewater to remove phosphorus.  With the chemicals, the smaller particles clump or 'floc' together into large masses.  The larger masses of particles will settle out in the sedimentation tank reducing the concentration of phosphorus by more than 95%.

Filtration

A variety of filtration methods are available to ensure high quality water.  Sand filtration, which consists of simply directing the flow of water through a sand bed, is used to remove residual suspended matter.  Filtration over activated carbon results in the removal of the following types of contaminants: non-biodegradable organic compounds, adsorbable organic halogens, toxins, color compounds and dyestuffs, aromatic compounds including phenol and bis-phenol A (BPA), chlorinated/halogenated organic compounds, and pesticides.

Although there are a number of different methods of membrane filtration, the most mature is pressure driven membrane filtration. This relies on a liquid being forced through a filter membrane with a high surface area.  Membrane filtration is designed to remove bacteria, viruses, pathogens, metals, and suspended solids.

Reverse osmosis

In the reverse osmosis process, pressure is used to force effluent through a membrane that retains contaminants on one side and allows the clean water to pass to the other side.  Reverse osmosis is actually a type of membrane filtration called microfiltration because it is capable of removing much smaller particles including dissolved solids such as salt.  This process is also effective at removing biological contaminants, metals, pharmaceuticals, pesticides, and endocrine disruptors.

Nutrient Removal

Nitrogen control  Ammonia in wastewater effluent can be toxic to aquatic life in certain instances.  By providing additional biological treatment beyond the secondary stage, nitrifying bacteria present in wastewater can biologically convert ammonia to the non-toxic nitrate through a process known as nitrification.  The nitrification process is normally sufficient to remove the toxicity associated with ammonia in the effluent.  Since nitrate is a nutrient, excess amounts can contribute to eutrophication in the receiving waters.  In situations where nitrogen must be completely removed from effleunt, an additional biological process can be added to the system to convert the nitrate to nitrogen gas.  The conversion of nitrate to nitrogen gas is accomplished by bacteria in a process known as denitrification.  Effluent with nitrogen in the form of nitrate is placed into a tank devoid of oxygen, where carbon-containing chemicals, such as methanol, are added.  In this oxygen-free environment, bacteria use the oxygen attached to the nitrogen in the nitrate form releasing nitrogen gas.  Because nitrogen comprises almost 80% of the air in the earth's atmophere, the release of nitrogen into the atmosphere does not cause any environmental harm.

Phosphorus control  Like nitrogen, phosphorus is a necessary nutrient for the growth of algae.  Phosphorus reduction is often needed to prevent eutrophication before discharging effluent into lakes, reservoirs, and estuaries.  Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal.  In this process, specific bacteria, called polyphosphate accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20% of their mass).  When the biomass enriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value.

Phosphorus removal can also be achieved by chemical precipitation, usually with salts or iron, alum, or lime.  This may lead to excessive sludge productions as hydroxides precipitates and the added chemicals can be expensive.  Despite this, chemical phosphorus removal requires a significantly smaller equipment footprint than biological removal, is easier to operate, and is often more reliable thatn biological phosphorus removal.

Sources:
http://en.wikipedia.org/wiki/Wastewater_treatment
http://en.wikipedia.org/wiki/Reverse_osmosis
http://www.epa.gov/OWM/primer.pdf
http://www.chemvironcarbon.com/en/applications/effluent-water-treatment/wastewater