HARNESSING THE POWER IN NITRIFYING SAND FILTERS

Transcription

1 HARNESSING THE POWER IN NITRIFYING SAND FILTERS Chan, T.F.*; Koodie, T.; Sloper, M.J. and Wiggam, R.W. Black and Veatch, 60 High Street, Redhill, Surrey RH1 1HS *Corresponding Author Abstract Black and Veatch installed and commissioned continuous blackwash nitrifying sand filters as the tertiary treatment stage at 10 sewage treatment works. The filters were installed to enhance the effluent quality of the existing works in order to meet the more stringent ammonia consents. Nitrifying sand filters have a number of advantages including their compactness and the simplicity of their operation. This paper has been written to present the key lessons learned and to provide guidance on achieving process optimisation during commissioning and operation. Keywords Continuous backwash filter, nitrifying sand filter, process commissioning, optimisation, backwash, air flow, dissolved oxygen profile, nutrient. Introduction Upflow continuous backwash filter technology was first developed in the 1970s for suspended solids removal. The filter operates in an upflow mode through the filter media, typically sand. The feed wastewater enters the lower section of the filter. The filter bed is moved continuously downwards, countercurrent to the upward feed wastewater flow, by means of an air lift created using compressed air at the base of the unit. The dirty sand/solids mixture is then passed, with added washwasher, through the sand washer and into a wash box. The flow out of the wash box, and hence the washwater flow, is controlled by an outlet weir which is always lower than the filtrate outlet. In the wash box, the denser cleaned sand is separated from the lighter solids and returned to the top of the filter bed. The solids laden washwater is diverted to the inlet works for treatment. The key advantages the continuous backwash filter offers include compact installation, continuous treatment and continuous dirty backwash water production. The process configuration has been further modified to incorporate tertiary nitrification or denitrification. In a nitrifying sand filter (NSF), air is added to the filter bed through aerators. The schematic of a typical continuous backwash nitrifying sand filter is shown in Figure 1. For a denitrifying sand filter, a carbon source is added to the incoming wastewater. These process configurations have since been widely applied in wastewater treatment worldwide (Feldthusen, 2004).

2 Figure 1: Schematic of a nitrifying sand filter (courtesy of Hydro International). Between 2010 and 2015, the fifth Asset Management Plan (AMP5) period, nitrifying sand filters were installed at 10 sewage treatment works in England. A summary of the scope of supply is presented in Table 1. Table 1: Summary of scope of NSF supply Parameter Feed Flow NH3-N discharge consent Design NH3-N load No of NSFs installed per site Range of NSF bed depth Range of NSF filter surface area Range of NSF filter volume Range Humus tank effluent from trickling filter works l/s 3-6 mg/l (95 th percentile) kg/day 2 8 units 2 5 m 5 7 m 2 / unit, m 2 / work m 3 /unit, m 3 /work The objective of this paper is to present the key lessons learned and to provide guidance on achieving optimum treatment performance. Commissioning and Operational Experience Air Distribution System Modifications Significant variations in the bubble patterns across some of the NSF units were observed during commissioning at a number of works. An example of this variation is shown in Figure 2.

3 Figure 2: Even air distribution vs. uneven air distribution. The larger surface area (7m 2 /unit) NSF units with rubber air hose distributors were installed at these works. When one of the units was emptied, it was found that the air hose in the outermost ring was stretched, as indicated by the movement of the fixing clamps away from the hose support, and showed sign of folding (Figure 3). The deformation of the hose may have occurred during sand media loading or due to the weight and continuous movement of the sand during operation. As a result of the deformation, the air supply to part of the filter was cut off. Clamps have moved inwards Figure 3: Deformed hose in the outer air distribution system. The air distribution systems of the affected works were subsequently modified (Figure 4). A stainless steel support was provided below the outer ring of hose to each unit. More fixing clamps were added to limit air hose movement. An additional line was also introduced to supply air to both ends of the air distribution hose and remove the single point of failure. The bubble pattern returned to normal immediately after the modification and has remained so since that point.

4 Figure 4: Modified air distribution system. Solids Accumulation and Remediation After the air distribution systems had been modified, air patterns in the NSF units at a few of the works soon deteriorated again. Upon detailed inspection, it was noted that the air lift in some of the units were not performing sufficiently and resulted in stalled sand bed movement. Attempts were made to restart the air lift by pulsing air through the system. When the air lift was restarted, the initial dirty washwater contained a high concentration of solid sludge mass. After an extended period of high rate washing, the solids concentration in the washwater decreased and the air distribution pattern returned to normal. At one of the affected works, air lifts in the NSF units could not be restarted by air pulsing and other attempts. When the units were drained for inspection, sludge mass as well as foreign objects were found, having accumulated at the base of the units affected (Figure 5). Figure 5: Sludge mass at the base of a NSF unit. Once the sand was cleaned and replaced, normal operations could then be resumed. Therefore, regular monitoring of sand movement, washwater flow rate and washwater quality would ensure the backwash system is performing effectively. Although 6mm auto-backwash filters have been provided upstream of the NSF units to prevent blockage of the air lift system and the sand bed by coarse solids, it is necessary to ensure the solids concentration and solid load in the feed are within the design limits. For example, more operator attention will be required when ferric dosing is used for phosphorous (P) removal upstream. The

5 NH3-N % Removal Air Flow (l/min) resulting solid material in the NSF feed can be more dense and can potentially accumulate at the base of the unit. In some cases, this solid material may need to be removed by increasing the air lift or by occasional pulsing of the air lift. More importantly, steps must be taken to prevent the humus tank sludge blanket from spilling downstream and clogging the NSF units. Effect of Air Flow on Treatment Performance Figure 6 shows the percentage NH3-N removal during several sampling periods for one of the NSF units. Both lab-tested and in-situ test kit results are included. It can be seen that the change in process air flow rate from 260 l/min to 400 l/min prompted a step improvement in NH3-N removal efficiency. This increase in the removal percentage is clear in both composite and spot samples. When the air flow was reduced, the removal efficiency dropped. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Jan Feb Mar Apr May Jun Jul NH3-N Removal % (Test Kit Spot) NH3-N Removal % (Test Kit Comp) Process Air NH3-N Removal % (Lab Spot) NH3-N Removal % (Lab Comp) Figure 6: Time-line showing process air to NSF and ammonia-n removal efficiency. Figure 7 shows the information in Figure 6 by plotting percentage NH3-N removal against process air flow rate. At the design air flow rate of 260 l/min, NH3-N removal efficiencies between 39% and 65% were achieved. Increasing the air flow rate appears to show a trend in increased removal efficiency. At the highest air flow tested, 400 l/min, the load removal was higher varying between 61% and 76%.

6 NH3-N % Removal 200l/min 230l/min 260l/min 400l/min 80% 70% 60% 50% 40% 30% 20% Process Air Flow Rate (l/min) Figure 7: Ammonia-N removal percentage versus process air flow rate. These results demonstrate the impact of increased air flow rate on the treatment of ammonia. While higher air flows do not guarantee greater removal of ammonia, an improvement in removal performance is indicated. Dissolved Oxygen Profile Dissolved Oxygen (DO) concentrations were measured within the same NSF unit through the use of a modified solid-state DO probe. The probe was encased in a perforated protective shell and attached to an aluminium pole. The shell prevented the ingress of sand, while allowing fluid to move freely to the probe. DO measurements were taken during operation by lowering the modified probe into the filter, stopping at incremental depths within the sand bed to allow the DO reading to stabilise. Measurements were taken in four regions around the NSF unit at North, South, East and West, approximately halfway along the filter radius from the centre.

7 Depth in Sand Bed (m) Depth in Sand Bed (m) Depth in Sand Bed (m) Depth in Sand Bed (m) Dissolved Oxygen Concentration (mg/l) Dissolved Oxygen Concentration (mg/l) NORTH 4.0 EAST Dissolved Oxygen Concentration (mg/l) Dissolved Oxygen Concentration (mg/l) l/min l/min 200l/min WEST 4.0 SOUTH Figure 8: DO profile at four locations around the NSF at different process air flow rate. Figure 8 shows the DO profiles through the filter bed at different process air flow rates and in the four regions. At -0.5 m, the results show the DO concentration within the liquid above the sand bed. 0.0 m indicates the upper surface of the sand bed. Subsequent measurements are at the indicated depths below the surface of the bed. Prior to this test, the standard procedure for ensuring that adequate DO is supplied for nitrification has been to measure the concentration in the liquid at the top of the filter, i.e. the filtrate. Barter and Smith (2007) indicated that a DO concentration of 6 mg/l in the filtrate is indicative of sufficient oxygen being present throughout the bed. The results presented in Figure 8 have demonstrated that the DO measurements at the top of filter or in the filtrate do not necessarily represent the DO concentration within the filter bed. DO concentration is known to be limiting to the growth of nitrifying bacteria below 2-4 mg/l Halling- Sørensen and Jorgensen (1993). The results in Figure 8 show that under conditions of lower process air flow (below 300 l/m) certain sections of the filter were very low in dissolved oxygen. These zones of low DO concentration were likely to have lower nitrifying activity thus to contribute to a lower

8 removal efficiency. This concurs with the lower NH3-N removal efficiency presented in Figure 6 and Figure 7 when the process air flow rate was at 200 and 230 l/min. The zones of lower DO concentration were more prominent in the West regions of the filter bed when the air flow rate was at 200 and 250 l/min. Other sections maintained higher DO concentrations even at process air flow rates below the design level. This indicates that the supply of process air was less evenly distributed at lower air flow rates. Alkalinity and Phosphorous as Nutrient At works where chemicals such as ferric compounds are added to enhance primary treatment performance or for phosphorous (P) removal, it is necessary to ensure sufficient residual alkalinity is available in the NSF feed to meet nitrification requirements. When the P consent becomes more stringent, there is a drive to dose more chemical to ensure the effluent quality meets the discharge consent. Chemical addition can potentially be too efficient in removing P from the wastewater and the residual P concentration can become limiting for tertiary nitrification processes. Nordeidet, et al. (1994) reported tertiary nitrification in a biofilm reactor was limited when influent P concentration was lower than 0.15 mg PO4-P/l. Therefore, the chemical dosing regime should be reviewed against the actual diurnal P loading pattern and the influent P level should be monitored regularly to ensure there is sufficient P available in the NSF feed water. Recommendations Based on our commissioning and operation experience, NSF performance can be optimised by the following: Ensure the air distribution system has been designed and constructed to suit the configuration of the unit and operating environment. Gross solids in excess of the design load, such as humus tank blanket spill, should be prevented from entering the NSF units. Effectiveness of the sand washing system should be checked by regular monitoring of sand movement, washwater flow rate and washwater quality. Increased air flow can improve the uniformity of air distribution across sand bed, while dissolved oxygen concentrations in the filtrate do not necessarily represent the DO concentration within the bed. Increasing air flow may improve treatment performance and sufficient air must be provided to the NSF which may be above the theoretical aeration rate to optimise treatment performance. At works where chemicals are dosed to enhance primary treatment performance and/or P removal it would be prudent to ensure alkalinity and nutrients (P in particular) are available for nitrification. Acknowledgement(s) The authors would like to thank the project team for their contributions, observations and suggestions during the operations of the unit. References Barter, P., Smith, J. (2007). Using Tertiary Aerated Sand Filters for Ammonia Removal. In: European Water & Wastewater Management Conference. Newcastle upon Tyne, UK Sep 2007.

9 Feldthusen, F. (2004). Continuous Sand Filters - Tertiary WWT and Other Applications. In: SAWEA Workshop. Dammam, KSA. 22 March Halling-Sørensen, B., Jorgensen, S.E. (1993). The Removal of Nitrogen Compounds from Wastewater. Elsevier Science. Nordeidet, B., Rusten, B. & Ødegaard, H. (1994). Phosphorus Requirements for Tertiary Nitrification in a Biofilm. Wat. Sci. Tech., Volume 10-11, pp