“The market for water reuse is on the verge of major expansion.”
That is the judgment of Global Water Intelligence (GWI) in its 2009 report on global water reuse. Between 2009 and 2016, it is expected that capital expenditure on advanced water reuse will grow at an annual rate of 19.5 percent. GWI cites four major factors creating growth in the water reuse sector:
- Pressure on the world’s water resources driven by climate change and population growth
- Growth of cities, creating greater stress on water resources and sanitation systems
- Environmental concerns have curtailed other solutions to water scarcity, including desalination and the construction of large dams
- New, proven technologies that have been proven to safely treat reclaimed water to be blended in reservoirs or aquifers for potable purposes
GWI also notes a growing use of reused water in applications other than the traditional agricultural market. Increasingly, urban water reuse is helping to reduce urban water stress and providing a higher return on investment to users of water reuse technologies.
Water shortages in Spain spur increase in reuse and recycling solutions
Among EU countries, Spain is one of the most severely affected by water shortages. Population growth, economic expansion, tourism and agriculture are the major drivers for rising demand. After investing heavily in national wastewater treatment and sanitation and purification plans, the Spanish government will put forth a new plan for water reuse and recycling in 2010. As a result, reuse and recycling is expected to increase from 368 million m³/yr by the end of 2009 to one billion m³/yr by 2015. The primary uses for reused water in the country are, in order of volume usage:
- Agriculture (75%)
- Recreation/golf courses (12%)
- Urban watering/cleaning (6%)
- Aquifer recharge (4%)
- Industrial (3%)
As Spain seeks to deal with its water shortage through reuse and recycling technologies, the experiences of other countries – such as the United States – can be instructive.
Successful reuse strategy in the U.S.: Denitrification
Untreated wastewater contains a number of pathogens and chemical constituents that are removed using a multiple-treatment approach involving primary, secondary and advanced/tertiary treatment methodologies. In the United States, which has the second largest country-installed reuse capacity behind China, deep bed denitrification has been a popular tertiary treatment choice since the technology was patented in 1979.
As total maximum daily loads for nutrient discharges have been developed and further revised by U.S. federal and state agencies to address water quality concerns, deep bed denitrification filters have proven to be a highly effective treatment technology used by wastewater plants to meet low total nitrogen (TN) limits. The technology of combining denitrification and solids removal in a deep bed filter process has helped to dramatically improve wastewater quality at treatment plants across the country.
U.S. wastewater treatment plants have been designed to convert ammonia via aeration into nitrate-nitrogen (NO3-N), excess levels of which can cause algae blooms and other oxygen-depleting growth in rivers, lakes and other water bodies. Nitrates also can be harmful for human consumption if introduced into drinking water supplies. When full nitrogen removal is required, biological denitrification is a technology frequently used at U.S. wastewater plants. During this form of denitrification, nitrate-nitrogen is biologically converted into nitrogen gas, thus playing an integral role in maintaining the integrity of the wastewater treatment plant’s receiving waters.
Denitrification is the biological process by which nitrate is converted to nitrogen and other gaseous end products. The denitrification process requirements are: a) nitrogen present in the form of nitrates, b) an organic carbon source and c) an anoxic environment. Denitrification can be achieved through chemical or biological methods. Specific to fixed-film technology, denitrification configurations are typically available as downflow or upflow filters. Both configurations require the addition of methanol or another readily biodegradable carbon source to wastewater ahead of the filter to enable denitrifying bacteria to grow.
Filtering liquids through deep beds of porous granular media to improve their clarity is widespread in municipal and industrial practice in the United States and is often used in tertiary wastewater filtration for reuse. Additionally, the removal of nutrients provides advanced wastewater treatment quality effluent. The TETRA® Denite® process from Severn Trent Services is one example of an economic and efficient use of deep bed filtration technology in the denitrification process.
As both a bioreactor and effluent filter, the TETRA Denite system combines deep bed filtration and fixed-film biological denitrification to achieve a high level of process synergy. Simultaneous removal of total suspended solids (TSS) and nitrate-nitrogen achieves 1 ppm nitrate-nitrogen and 3 ppm TN or less.
How the TETRA Denite process meets the most stringent requirements for tertiary treatment
Fundamental to the TETRA Denite process is the specially sized and shaped granular media used in the fixed-film biological filters. The high solids loading capacity of the media is ideal for retaining biological solids produced by the denitrification process, and the powerful backwash of the Denite filter system removes these solids periodically. The surface area of the 2-3 mm-diameter sand particles is very large, providing 1,000 m²-per-cubic-meter contact between the wastewater supply and the biomass. The media allows for heavy capture of solids – at least one pound of solids per square foot of filter surface area before backwashing is required. The high solids capture permits extended operating periods and easily handles peak flow or plant upsets.
During the fixed-film biological denitrification process, wastewater is forced to flow around nitrogen gas bubbles that accumulate in media voids in the filtration vessel, improving biomass contact and filtration efficiency. Effective removal of nitrate-nitrogen is undertaken by introducing a carbon source, such as methanol, using the TETRAPace® automatic dosing control. Methanol, a food source for microorganisms in the Denite system, is stored and fed automatically. This dosing control scheme is based on an influent flow signal combined with an influent and effluent concentration analyzer. An alternative to this system is one incorporating either a flow-paced or feed-forward or feedback system. But, TETRAPace is far more efficient. The advantages of tighter methanol control can be significant if the plant has a stringent BOD limit in combination with a low TN limit. Under these conditions, the tighter control and reduced risk can be a critical component in ensuring the plant meets limits reliably. The accuracy of the proprietary algorithm used to feed methanol during the denitrification process enables TETRAPace to yield significant savings of up to 30 percent in carbon consumption costs while guaranteeing effluent quality with a “no net TOC pickup across the filter” guarantee. TETRAPace can also be used simultaneously for auto dosing of metal salts for orthophosphate removal.
The Denite system employs a “bump” operation to remove or purge accumulated gas – nitrogen or CO2 – that can build up in the filter media. If desired, this bumping can be accomplished without removing the reactor from service using SpeedBump®, which applies backwash water to the bottom of the filter, releasing the entrapped gas into the atmosphere and reducing headloss.
An added benefit of the Denite process is the removal of phosphorus, which is consumed in the cell wall biology of the biomass. The trapped solids are backwashed out of the filter by a simultaneous injection of air and water and returned to the upstream biological treatment units at the end of each cycle. By operating down flow, excellent levels of solids removal are achieved, eliminating the need for additional effluent polishing filters.
As countries throughout the world struggle to address the effects of population growth, economic development and other factors contributing to water shortages, reuse strategies are rapidly gaining favor as cost effective, environmentally conscious and sustainable solutions. While new and improved wastewater treatment technologies are continuously being developed, fixed-film biological deep bed denitrification filters continue to set the standard for meeting the lower, more stringent, total nitrogen limits required for water reuse.
For more information, e-mail info@severntrentservices.com.
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