Water Journal : Water Journal December 2011
my point of view rose a new interest in the development of side-stream fermentation of return-activated sludge convinced the company to convert all their plants to mainly biological operation, with an estimated saving of £4m per annum. Phosphorus removal to very low levels requires that chemicals be used for polishing, but even then substantial savings by the combination of biological and chemical systems are possible. In a small plant in the state of Colorado, the consumption of chemicals was reduced to a molar ratio of Alum to P of around 0.4, by maximising biological phosphorus removal for reducing the influent phosphorus from 9mg/l to an average of 0.025mg/l. While the wastewater characteristics were not favourable for BPR, some in-plant fermentation of mixed liquor supplied sufficient volatile fatty acids to reduce the final clarifier effluent TP to less than 0.5mg/l. A recent study in a ski resort in the same state showed that it was possible to reduce ortho- phosphorus to less than 0.03mg/l by using a similar method. The Question of Sustainability An exciting concept is the development of fuel cells in which the biological reactions are driven by using a cathode as electron acceptor instead of oxygen, thereby generating energy rather than using energy. However, in spite of the fact that aeration is one of the most important energy users at a treatment plant, the actual energy consumption of a well- designed and operating biological nutrient removal plant is less than 35kWh per person per year -- and while great strides have been made in fuel cells, much more research breakthroughs will be necessary to justify the capital cost for saving that energy. The new catch phrase is "sustainability", but it is not always clear what the term means, even though definitions abound. It is used, for example, when designing a plant to produce ultra- low nitrogen or phosphorus at huge cost and carbon footprint. A question was asked in a recent workshop: "Should sustainability not start with the regulators?" What is the point of driving down effluent phosphorus to less than 0.01mg/l at great cost when the non-point sources form a large component of the phosphorus discharged to a lake? Should the phosphorus limit not be set at a percentage of the diffused sources? Why spend massive amounts of money on reducing the phosphorus at point sources to below, say, 2--5% of that of the diffused sources? And why remove nitrates when in many instances it is beneficial to the health of the reservoir? When a reservoir has been shown to be phosphorus limiting and it gets phosphorus from diffused sources, discharging some nitrates may prevent the growth of nitrogen-fixing cyanobacter with their associated toxins, while preventing the release of phosphorus in the hypolimnion. These are questions that need serious consideration if we don't want to make a mockery of sustainability studies. Westbank process at Wilson Creek in Texas. Contact a Hach Pacific office near you: 1300 887 735 www.hachpacific.com Innovative Process Instrumentation Integrated Lab Solutions and Chemistries Expert Support Dependable Service Products. Support. Expertise. Buy direct from Hach Pacific and receive access to the largest offering of the highest quality lab and process water analytics as well as outstanding service and application support.
Water Journal April 2012
Water Journal November 2011