Deep-bed Detox

An established technology is helping remove harmful nutrients from municipal wastewater

As total maximum daily loads (TMDLs) for nutrient discharges have been developed and further revised by federal and state agencies over the years 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. Patented in 1979, 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.

Ammonia, Nitrogen and Phosphorus
On a daily basis, nitrogen consumed as protein in our food, but unused in our bodies, is excreted in the form of ammonia. Since ammonia and total nitrogen are highly toxic to fish and other animals, levels in wastewater treatment and effluent discharges must be closely monitored to ensure the nutrient levels are not harmful to receiving streams.

Therefore, the wastewater quality-monitoring process is critical to ensuring effluent discharge levels that are safe for wildlife. Additionally, the monitoring of total nitrogen levels is used to document discharge levels against set regulatory limits -- a critical function since non-compliance can result in penalties to a wastewater facility.

Denitrification is the biological process by which nitrate is converted to nitrogen and other gaseous end products (NO3 --> N2). 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).

Wastewater treatment plants have been designed to convert ammonia, via aeration, into nitrate-nitrogen (NO3-N). Nitrate-nitrogen promotes plant growth, so excess levels can cause algal blooms and other oxygen-depleting growth in rivers, lakes, and other water bodies. In addition, nitrates can be harmful for human consumption if introduced into drinking water supplies.

Phosphate in wastewater also will encourage growth in rivers and lakes. Eutrophication causes algae to amass, impacting the ecological balance of local ecosystems.

When full nitrogen removal is required, one of the available treatment methods is biological denitrification. 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.

Deep Bed Filtration
Filtering liquids through deep beds of porous granular media to improve their clarity is a widespread municipal and industrial practice 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 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 1ppm nitrate-nitrogen and 3 ppm total nitrogen (TN) or less.

Fundamental to this 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 filter system removes these solids periodically. The surface area of the 2- to 3-millimeter diameter sand particles is very large, providing 1000m2 per cubic meter contact between the wastewater supply and the biomass. The media allows for heavy capture of solids, at least 1 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 this 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 methanol using automatic dosing control. Methanol, a food source for microorganisms in the 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 this system is far more efficient. The advantages of tighter methanol control can be significant if the plant has a stringent biochemical oxygen demand (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 methanol consumption costs.

"Bump" operation removes or purges accumulated gas -- nitrogen or CO2 -- that can potentially build up in the filter media. If desired, this "bumping" can be accomplished without removing the reactor from service using a process that applies backwash water to the bottom of the filter, releasing the entrapped gas into the atmosphere and reducing head loss.

An added benefit to the 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.
In an increasingly demanding and price-conscious industry, fixed-film biological denitrification technologies have proven their efficiency and cost-effectiveness at treatment plants across the country.

In an industry where new and improved wastewater treatment technologies are introduced at a rapid pace, fixed-film biological deep bed denitrification filters continue to set the standard for meeting lower, more stringent, total nitrogen limits. At treatment plants across the United States, the combination of deep bed filtration and fixed-film biological denitrification achieves a high level of process synergy, maximizing system economy and efficiency.

Pinellas County, Florida
In Pinellas County, Florida's most densely populated county, water conservation and efficient use of all water resources is facilitated by a state-of-the-art water reclamation facility. The South Cross Bayou Water Reclamation Facility (WRF) is a permitted advanced wastewater treatment facility (AWTF) using a tertiary treatment process to treat an average daily flow rate of 33 mgd. The South Cross Bayou Water Reclamation Facility cleans and treats wastewater to meet reclaimed water standards of 5-5-3-1. In Florida, advanced wastewater treatment standards require an effluent quality which is no more than 5 ppm BOD, 5 ppm TSS, 3 ppm (or mg/l) TN and 1 ppm total phosphorus (TP).

The wastewater treatment process is made up of four treatment stages: primary sedimentation followed by anaerobic, anoxic, and oxic zones; secondary sedimentation; and tertiary filtration with nutrient removal and disinfection treatment. During the primary treatment phase, large solids are eliminated and the bio-treatment phase uses micro-organisms to break down smaller solids. The secondary phase clarifies the wastewater. Unwanted nitrogen-based compounds and finely suspended particles are removed in the tertiary treatment phase. The final treatment stage involves disinfection with chlorine to eliminate disease-causing organisms.

While the goal of the facility is to use 100 percent of the reclaimed water for irrigation purposes in the surrounding area, the advanced treatment enables wastewater to be released into nearby Joe's Creek when it otherwise could not be recycled for use. In such instances, the reclaimed wastewater must meet permitted regulations and requires additional treatment prior to discharge into Joe's Creek. The wastewater is treated to remove chlorine and re-aerated to enrich it with additional oxygen.

Pinellas County has two wastewater treatment facilities, the W.E. Dunn WRF to the North and the South Cross Bayou WRF serving the southern part of the county. The 33 mgd Pinellas County South Cross Bayou AWTF has a peak flow rate of more than 66 mgd. In an effort to meet required standards, the facility uses twelve 9 foot, 8 inch-wide x 85 feet long deep bed filters with 6 feet of media each. The system also employs an automatic dose control system and bump process to conduct a complete system bump cycle without stopping flow to the reactors.

Wastewater to the South Cross Bayou WRF flows to a common headworks, traveling through suspended solids screens and teacups for grit removal before splitting into separate trains. Once separated, the streams pass through rectangular primary clarification, anaerobic, and anoxic zones, followed by fine bubble diffusers and secondary clarification. After secondary clarification, the streams are once again combined and pumped to the 10,000 square feet of filters. After denitrification, chlorine is injected and the stream is split for reuse where it is stored in three 10-million gallon tanks or dechlorinated for flow that will be diverted to Joe's Creek, which runs into the Gulf of Mexico.

A summary of the TETRA Denite performance at Pinellas County South Cross Bayou AWTF is detailed below.

Pinellas County South Cross Bayou AWTF
Average 2002 12.3 1.84 1.03 0.55 2.07
Average 2003 19.79 1.53 1.09 0.76 2.09
Average 2004 26.89 2.92 1.47 0.66 2.72
Average 2005 27.00 3.69 1.84 0.09 2.13

This article originally appeared in the 09/01/2006 issue of Environmental Protection.

About the Author

David C. Slack is the general manager of the Severn Trent Services office in Tampa, Fla. He has a BS in electrical engineering and an MBA in economics and management.

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