CBFs aid nutrient removal

Utilities continue to search for efficient and cost-effective methods to meet increasingly tighter effluent discharge permit limits.

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Using a continuous backwash filter for secondary effluent denitrification key to enhanced nutrient removal at Dutch wastewater treatment facility

Utilities continue to search for efficient and cost-effective methods to meet increasingly tighter effluent discharge permit limits. One effluent parameter drawing increased scrutiny is nitrogen, usually measured as either nitrate-nitrogen (NO3-N) or total nitrogen (TN). Several technologies, including continuous backwash filters (CBFs), can significantly reduce nitrogen levels. CBFs have already been used for years to remove solids from secondary effluent.

The Astrasand® filter is one such full-scale CBF that uses an innovative process control system to regulate the sand circulation rate to maintain an optimum biomass concentration within the media. The CBF technology is based on moving bed biofiltration (MBBF) that combines the benefits of upflow sand filtration for solids removal with the biological conversion of nitrates into nitrogen gas. The biological component resides in the voids and crevices of the sand medium. Manufactured by Paques B.V., the filter is marketed by Siemens Water Technologies in North America.

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Figure 1: Astrasand filtration process
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The Sand Circulation Process
In a typical operating scheme of a CBF, an influent pipe transports secondary effluent into the filter. Water enters the filter bed through the feed pipe and distributors. The dirty water flows upwards where solid particles are trapped within the sand filter media and purified filtrate is discharged from the top of the filter through the effluent pipe. As the water flows up, the filter bed continuously moves downward. An airlift forces the dirty media from the bottom of the bed up through a central airlift pipeline, around the top of which a sand washer is positioned. Sand particles fall through the washer. The intense scouring action within the airlift wash-box separates most of the contaminating biomass from the sand filter media. Dirty water is discharged from the top of the filter while washed, clean sand is deposited at the top of the media bed.

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Pumping station
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Denitrification
A CBF configured and operated as a bioreactor can achieve biological denitrification. Within the filter bed, anoxic conditions prevail that enable the denitrifying biomass to grow on the surface of the sand grains and in the pores of the bed. The sand filter media acts to support heterotrophic bacteria that use an external carbon source (typically methanol or acetate) injected into the influent stream to convert nitrates into nitrogen gas. In this so-called “moving bed biofilter”, or MBBF, the sand is continuously washed, and the excess biomass produced by this biological reaction is constantly removed. The airlift rate for media washing must be carefully controlled to ensure optimum biomass retention, especially under fluctuating hydraulic and nitrate loading conditions. Washing media too aggressively may reduce the effective bacterial population within the bed, resulting in inefficient biological activity and nitrate breakthrough. The process has been successfully applied for denitrifying secondary effluent to meet ever-stringent effluent criteria for total nitrogen, as illustrated below.

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Siemens Pro V controller with display
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De Groote Lucht WWTP Design
In the late 1990s, the 4,350 m3/h (27.5 MGD) full-flow treatment De Groote Lucht wastewater treatment plant (WWTP) in Vlaardingen, the Netherlands, had to amend its effluent treatment process to meet an annual average TN limit of 10 mg/L. The plant’s basic treatment scheme consisted of headworks screening and primary clarifiers, followed by two-stage aerobic treatment for BOD and ammonia removal. To meet the more stringent nitrogen discharge limits, De Groote Lucht considered expanding its original process or adding secondary effluent post-denitrification treatment. The plant selected secondary effluent filtration for its restricted footprint, easier planning and construction, and lower capital investment and operating costs. An Astrasand MBBF filter was installed as a post-denitrification stage to meet the new effluent limits.

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Because the discharge limits are related to annual average N-levels, it’s unnecessary to treat the full wet weather flow of 4,350 m3/h. Flows exceeding the maximum design flow bypass the filter and are discharged directly into the receiving stream. The filters’ required hydraulic capacity was determined to be 3,600 m3/h, with a specific hydraulic load of 15 m/h based on an expected denitrification efficiency of 88%. The installation consists of six concrete filter units, each with 40 m2 of filter area. The effective bed height for filtration is 3.6 m, resulting in a bioreactor volume of about 864 m3.

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The process was designed to achieve an effluent NO3-N concentration of less than 10 mg/L and a removal efficiency of greater than 88%, as well as effluent suspended solids concentrations less than 10 mg/L and a COD increase, due to overfeeding methanol, no greater than 20 mg/L.

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Process Control
Due to normal differential pressure between the bed headloss and airlift, an increased hydraulic load automatically results in higher sand circulation (backwash) rates without adjusting compressed air flow to the airlift. The sand circulation rate, therefore, can be considered largely self-regulating. In some cases, though, such as applications with broad variations in process conditions or with strict effluent limits, it may be useful to amplify this self-regulating function by employing automated control of the air flow to the airlift.

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The De Groote Lucht plant often experiences wide fluctuations in hydraulic loads. To achieve optimum filter performance and avoid nitrate breakthrough, a process control system was developed to maintain high biological activity within the filter at varying load conditions. Key to this control system, automatic adjustment of the sand recirculation rate is based on desired biomass content in the filter bed. The volume of solids and/or biomass present in the bed is directly related to headloss over the filter bed. The principle of this strategy is to maintain a sufficient amount of denitrifying biomass, measured mainly by headloss, in order to anticipate increasing nitrate loads during the day. Compressed air flow to the airlift is regulated by continuously measuring headloss and other important process parameters, such as feed flow and water temperature, and by using calibrated process algorithms within a local PLC.

Performance Data

The full-scale installation was completed in February 1999 and started up biologically in April 1999. Full denitrification was established in about four weeks. Results for a representative period in August 1999 show an average nitrate removal efficiency of 95.6%, with an average influent NO3-N concentration of 11.1 mg/L. Methanol dosing employs feed forward control based on continuous measurement of influent nitrate concentration and flow. This assures an efficient chemical feed with limited COD increase in the effluent, as indicated by the slight average COD increase in Table 3, with the feed COD measured prior to methanol addition. An on-line effluent COD analyzer has also been used for feedback control of methanol addition to avoid over-dosage. The total phosphorus showed a slight decrease in concentration across the filter, mainly due to uptake by the biomass.

Performance data was collected over the next three years of operation. The data indicates the filter successfully met the required effluent limit on an annual average basis, with the average filtrate NO3-N concentration of 2.5 mg/L and average nitrate removal efficiencies of 86%. Efficient methanol feed was also demonstrated. Allowing for an average 10% consumption of methanol to remove residual dissolved oxygen carried over from the secondary treatment process, the average specific methanol consumption of 3.3 mg CH3OH/mg N removed is well within the expected range. It also demonstrates efficient and effective organic carbon utilization by the denitrifying biomass the filter bed. This is very important as chemical costs for methanol are one of the biggest expenditures in operating a denitrification filter.

Conclusion

CBF is a well-established, effective and easy-to-operate secondary effluent solids removal process successfully used for many years worldwide. Growing pressure to reduce nutrient discharges into many U.S. watersheds caused some manufacturers to adapt CBFs as efficient nitrogen removal devices. By using the filter sand bed as an anoxic bioreactor and controlling the retention of the denitrifying biomass through the automation of the airlift, a conventional CBF becomes an MBBF, with exceptional capabilities to fulfill even the strictest of effluent nitrogen discharge limits.

Although data was not presented in this study, recent work has demonstrated that simultaneous nitrogen and phosphorus removal, through the addition of metal salt solutions, also can be accomplished using this technology, achieving effluent phosphorus levels below 0.15 mg/L while maintaining filtrate TN levels well below 3 mg/L.

Author’s Notes:

Anthony (Tony) J. Freed, Jr. , of Thomasville, Georgia, USA, is a product manager for Davco biological systems at Siemens Water Technologies. Contact: anthony.freed@siemens.com. Miguel A. Gutierrez, also of Thomasville, is filtration products manager for the Clarification & Filtration Product Group at Siemens Water Technologies. Contact: miguel.a.gutierrez@siemens.com. Coos Wessels, of Balk, the Netherlands, is a sales manager at Paques B.V. Contact: c.wessels@paques.nl. Content originally presented at the NCAWWA/WEA 85th Annual Conference held in Greensboro, North Carolina, USA, on 14 November, 2005.

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