Wastewater Treatment Basic Terminology: What Is a Denitrification Filter
A denitrification filter is a sewage treatment unit that integrates biological nitrogen removal and filtration functions, used to remove nitrate nitrogen (NO3-N) and suspended solids (SS), and is one of the most important structures in the advanced treatment of high-standard discharge sewage plants. [2] [6] Its core process is the organic combination of denitrification and deep-bed filtration. Under anoxic conditions, denitrifying bacteria use a carbon source to reduce nitrate nitrogen and nitrite nitrogen to nitrogen gas (N2) for discharge, while simultaneously intercepting suspended solids through filter media (such as quartz sand). The world's first denitrification filter was born in 1969, and for nearly 40 years hundreds of systems have been operating worldwide. [2] According to differences in structure, operation mode and filter media, denitrification filters can be divided into various types such as denitrifying deep-bed filters, activated sand filters, and denitrification biofilters (DNBF). [6] According to hydraulic flow regime, denitrification filters are divided into two forms: up-flow and down-flow. The up-flow denitrification filter form: wastewater flows from bottom to top through the distribution layer, support layer, filter layer, and clear water layer. The down-flow denitrification filter form: wastewater enters the filter media zone from the distribution tank at the top of the filter, and the filter is divided from top to bottom into the distribution zone, filter zone, support layer, and effluent collection zone. [8]
A typical representative is the denitrifying deep-bed filter, which mostly uses 2-3 mm quartz sand as filter media, and the filter bed depth is usually 1.83 m, which can ensure the effluent SS is below 5 mg/L. Without chemical dosing for phosphorus removal, it can meet the effluent quality requirements for BOD, SS, TN and TP; when chemical phosphorus removal is performed, the effluent TP can be below 0.3 mg/L. The main components include: filter media (quartz sand), gravel layer, filter bricks, air intake pipe, weir plate, control system, valves, carbon source storage and supply system, backwash pump, and blower. [2]
The world's first denitrification filter was born in 1969. For nearly 40 years, denitrification filters have had hundreds of systems operating normally worldwide. [2] According to differences in fillers and functions, those currently in common use include BAF (biological aerated filter), denitrification filter, and tower biological filter. [4-5] In recent years, autotrophic denitrification filters have emerged as a relatively new advanced denitrification process. This process uses autotrophic microorganisms, with inorganic substances (such as sulfur and iron) as electron donors, without the need to add organic carbon sources, avoiding the problem of increased effluent COD and reducing operating costs. [3] This technology has been widely applied in China, with a total sewage treatment capacity exceeding 1 million tons/day. Denitrification filters have emerged in different tank types such as up-flow and down-flow, and are combined with intelligent control systems and multi-stage AO processes. [8] [11-12] [20]
According to differences in structure, operation mode and filter media, denitrification filters can be divided into multiple types. They mainly include denitrifying deep-bed filters, activated sand filters, and denitrification biofilters (DNBF). Among them, the denitrifying deep-bed filter is developed on the basis of the traditional V-shaped filter; the activated sand filter integrates coagulation, clarification and filtration; and the DNBF is improved on the basis of the biological aerated filter [6].
The filter media is usually 2-3 mm quartz sand or ceramsite, etc. [2] [7-8] The empirical design hydraulic load value is 0.5-3 m3.m-2.h-1. The suitable environmental conditions for denitrifying bacteria are pH 6-8, water temperature 20-35 degrees C, and dissolved oxygen concentration <=0.5 mg/L. In engineering, the carbon-nitrogen ratio (C/N) is generally required to be >=5:1. The carbon source type and dosing amount (such as methanol, sodium acetate), hydraulic retention time (HRT), dissolved oxygen (DO), pH value, and temperature are the keys to ensuring the denitrification effect [8] [18].
According to the hydraulic flow regime, denitrification filters are mainly divided into two forms: up-flow (rising flow) and down-flow. In the up-flow filter, wastewater enters from the bottom and flows out from the top, and from bottom to top are the distribution layer, support layer, filter layer, and clear water layer. The down-flow filter structure is similar to the V-shaped filter; wastewater enters the filter media zone from the distribution tank at the top of the filter, and from top to bottom are the distribution zone, filter zone, support layer, and effluent collection zone [8]. In addition, the up-flow filter has the characteristics of higher denitrification efficiency, phosphorus removal function, and relatively low carbon source consumption, and its water and air distribution system design helps alleviate the problem of filter media clogging [9].
To ensure the operating effect, the denitrification filter needs to be regularly backwashed with combined air and water to restore head loss and discharge aged biofilm [8]. The backwash wastewater generally returns to the front-end treatment unit, and its water consumption usually does not exceed 4% of the total water volume of the treatment plant [2] [7]. New-type filters mostly use optimized filter bricks for water and air distribution to improve backwash uniformity and efficiency [8].
Denitrification filters are mainly applied to the advanced treatment unit of sewage plants, used to further remove total nitrogen (TN), suspended solids (SS) and total phosphorus (TP) to meet high-standard discharge requirements (such as Class 1A standard, quasi-Class IV standard). [6-7] [9] In the process flow, the denitrification filter can be used as a post-denitrification process placed after the secondary biological treatment, or connected in series with the nitrification filter as a pre-denitrification process, and is especially suitable for water plants with small treatment volumes. [8] According to structure and hydraulic flow regime, it is mainly divided into two tank types: down-flow (such as Denite filter) and up-flow, and the engineering design needs to be selected in combination with factors such as geology, water quality, and operation management. [11]
The denitrification process requires organic carbon source as an electron donor. Common carbon sources include methanol, sodium acetate (acetic acid), etc. [2] [10] Research shows that when methanol is used as the carbon source, the denitrification effect is more stable, with less nitrite accumulation in the effluent, and it may be more economical; sodium acetate is also an effective carbon source choice. [10] [17] In practical applications, comprehensive selection should be made based on water quality, safety and economy. [10]
In addition to traditional heterotrophic denitrification, autotrophic denitrification filters (such as the sulfur autotrophic denitrification advanced nitrogen removal technology) use sulfur-based composite active biological carriers and other inorganic filter media, require no external organic carbon source, and are suitable for low carbon-nitrogen ratio wastewater, with advantages such as energy saving, low sludge production, and no secondary pollution risk. [3] [12] This technology has engineering applications at home and abroad, with a total nitrogen removal rate >=90%. [12]
Typical engineering cases include the upgrading and renovation project of Shenzhen Henggang Water Purification Plant (using up-flow denitrification filter), Xiehe Regenerated Water Plant (using 'high-efficiency sedimentation tank + deep-bed denitrification filter'), the renovation of the second sewage treatment plant in the high-tech zone, and the nitrate nitrogen wastewater treatment project of Suntech Power Co., Ltd. [9] [13-15]
Through operation optimization measures such as intelligent backwashing and precise carbon source dosing, the denitrification filter can continuously improve treatment efficiency and achieve energy saving, consumption reduction and stable compliant operation [20-21].
As a key advanced treatment process for achieving high-standard discharge (such as Class 1A and quasi-Class IV water standards), the denitrification filter effectively improves the effluent quality of sewage plants. Its application has contributed to the upgrading and renovation projects of many sewage plants, meeting increasingly strict environmental protection requirements [14-16].
As a key technology for reclaimed water advanced treatment, the denitrification biofilter is widely used. Related research focuses on improving denitrification efficiency, reducing costs, and dealing with emerging pollutants [18].
Carbon source and filter media are key factors affecting the efficiency of denitrification filters. Research shows that different carbon sources and filter media have a significant impact on the denitrification effect. For example, some studies compared the performance of quartz sand and biological ceramsite as filter media when using methanol and sodium acetate as carbon sources, and found that methanol as a carbon source gave more stable effluent and almost no nitrite accumulation; at the same time, biological ceramsite as a filler was superior to quartz sand in biomass and pollutant removal effect [17].
Optimization of operating parameters is a research focus for ensuring the efficient and stable operation of the filter. Related studies examined key parameters such as the filter biofilm start-up method, optimal carbon-nitrogen ratio (C/N) and hydraulic retention time (HRT). For example, a filter using sodium acetate as carbon source started up faster (stable in 7 days), while one using methanol as carbon source needed 9 days; during stable operation, the carbon-nitrogen ratio required to achieve maximum denitrification efficiency was about 4.5-5.5 [18].
To break through the dependence of traditional heterotrophic denitrification on externally added organic carbon sources and to deal with emerging pollutants, researchers have explored new processes and materials [3] [19]. For example, autotrophic denitrification filters use inorganic substances such as sulfur and iron as electron donors, require no externally added organic carbon source, and can avoid secondary pollution and reduce operating costs [3]. In addition, some studies constructed biomass-iron-based denitrification filters to explore their performance in simultaneously removing nitrogen and emerging pollutants such as antibiotics [19].
For large-scale actual projects, refined research on operation control strategies is crucial to ensure stable compliance of effluent quality. Related research optimizes and regulates core factors such as filtration rate, backwash intensity, influent dissolved oxygen and carbon source dosing to improve the operation level. For example, some studies adopted strategies such as balanced load, reinforcement and renovation, backwash correction, oxygen elimination and pressure stabilization, and precise methanol dosing for the denitrification biofilter of a large reclaimed water plant, achieving precise control of effluent total nitrogen [21].
