Focus on nitrification

Hans Wouters of Paques and Wesley Jones of Aquabio discuss the implementation of an ammonia removal system at a Somerset WwTW

Many WwTWs in the UK are now facing pressures, namely through AMP3 and AMP4, to upgrade their plants so stringent discharge consents can be met. Ammonia consents as low as 2.0mg/l have been implemented by the Environment Agency (EA) and with the ongoing implementation of the Habitats Directive, and the forthcoming Water Framework Directive (WFD), there may be more requirements for similar standards in the future.

Difficulties are encountered at older sites, which utilise trickling filters, where the only option to meet consent is to increase the aerobic volume of the bioreactor. Footprint limitations and large capital costs mean construction of extra trickling filters is frequently not an option. Other strategies include the addition of a SAF or BAFF, but commonly a sand filter or reed bed has to be installed in addition to remove TSS and associated BOD. This again can mean high capital costs for upgrading the plant. This article looks at a recent example of installing an aerated sand filter in which ammonia, TSS and BOD can be removed in one single process, therefore saving on land space and capital expenditure. The existing infrastructure of the original plant can also remain virtually unchanged as the Astrasand system is an add-on tertiary process. Furthermore, the sand filters are modular in design, making future upgrades of the plant relatively easy.

Biofiltration Process

The Astrasand unit is a moving bed filtration unit (see Figure 1). This process has already proved to be effective for final removal of TSS, BOD, nitrates and phosphates. Wastewater is fed into the filter at (1) and then into the filter bed at (4) through the feed manifold (2) and distributors (3).

The water is filtered as it passes through the sand bed with the treated filtrate discharged in the upper part of the unit at (5). The sand bed is continuously moving downwards as the water rises. The dirty sand (6) is abstracted from the bed and raised to the upper part of the filter, after which it is released back on top of the sand bed (7). The sand recirculation is based on the airlift principle forcing a mixture of dirty sand and water upwards through a central pipeline (8). The intensive scouring separates the impurities from the sand particles. At the top of the pipeline the air is released, the dirty (wash)water discharged (9) and the sand settles in the washer (10).

As the sand passes through the washer labyrinth the particles are finally rinsed by a small amount of clean filtrate through the washer counter-currently. The filtrate flow is generated by a difference in discharge level between the filtrate (11) and the washwater (9). The required oxygen uptake for the nitrification process is supplied with compressed air distributed homogeneously over the filter area, in order to ensure an even gas flow rate. The small air bubbles migrate through the sand bed at a low-speed, which promotes the efficient uptake of air by the nitrifying micro-organisms. Biomass grows on and in between the grains of sand. The growth of biomass is balanced against the continuous discharge of biomass into the washer at the top of the filter. A control mode has been developed, particularly suitable for optimising the biofiltration performance of the moving bed. The Astracontrol continuously adjusts the sand recirculation rate to maintain a fixed amount of solids/biomass within the filter.

Thus the process is optimised and is able to deal with variations in load (both in terms of influent quality and quantity). Nether Stowey WwTW in Somerset consists of primary settlement, trickling stone media filters and humus tanks. The works needed to be upgraded in order to meet the stringent consent levels with respect to TSS, BOD and ammonia implemented on the April 1, 2004. The 95 percentile levels are set at 18, 9 and 2mg/l respectively. Peak filter feed levels are projected to be 35, 25 and 9.5mg/l respectively. Further to the biofiltration schemes already installed at Foulridge and north Harrogate WwTWs (both Yorkshire Water), Wessex Water decided to install a nitrifying moving bed filter plant at Nether Stowey designed to treat 122m3/hr.

The plant was constructed at the end of 2003 and commissioned in the beginning of 2004 (Figure 2). A total of three 5m2 filters have been installed, of which two are able to handle full-flow to treatment. During the commissioning period the filters were tested at artificial loading rates.

Under these non-limiting conditions, a rapid nitrifying capacity uptake was created using the Astracontrol filter control, even under conditions of low temperatures and high-feed water turbidity. Figure 3 indicates the actual nitrifying uptake capacity during start-up of the plant. The start-up was executed during the winter period, with water temperatures in the range of 6-10°C.

To promote the start-up procedure the biofilter was seeded with activated sludge from another WwTW. After a month, the exponential growth of nitrifying bacteria resulted in a rapid uptake of removal capacity up to the design loading capacity of 0.60kg N/m3 of bed volume/day.

Figure 4 illustrates a stable nitrification process after the plant start-up in the first two weeks of March. Consistent ammonia levels of less than 2mg/l NH4+-N were being achieved. This data was gathered from the Scopex data system using Envitech's double buoy PBS-1 ammonia monitors for both filter feed and filtrate. From practical experience the online analysers always need to be compared to the lab results in order to evaluate the consistent accuracy of the data.

However, the online analysis is a good tool to monitor and judge the filter performance under varying process conditions seen over this time period. Furthermore, it is essential to calibrate the analysers on a regular basis, especially for lower NH4+-N concentration (<2mg/l), in order to create stable and reliable data.

The nitrifying sand filters were also designed to remove TSS and BOD. Figure 5 illustrates typical TSS removal over a period of time. Although the sand bed is being partially disturbed by the aeration process, the Astrasand filter is able to remove suspended solid concentrations to well below the consent level. BOD analysis during this period also showed consistent low levels for the filtrate (below detection limits).

Varying conditions

With a biologically enhanced process the growth of nitrifiers depends on actual loading conditions. If for a longer period the ammonia loading is low, the risk of losing activity is present. Therefore, the process operating strategy should be focussed upon maintaining biomass, the filter control is a vital tool to achieve this because it helps maintain biomass in the filter at periods of low-loading rates. Figure 6 shows a typical trend, based upon online analyser data, highlighting the system's response to varying feed ammonia concentrations. The peaks in the humus tank effluent ammonia concentrations were partly due to the frequent manual desludging activities on site.

The Astrasand filter was able to cope with spikes in ammonia and keep filtrate levels well below the 2mg/l consent level. Astrasand moving bed biofiltration is a reliable, compact and cost-effective technology for the combined removal of ammonia, BOD and TSS. Low consent levels can be reached, while the process is robust and easy to handle. Even under varying process conditions, low filtrate levels can be achieved due to the process optimisation provided by the Astracontrol system, which promotes the retention of biomass within the filter. As a result, the unit is able to maintain high biological activity, even if the system is under loaded for a period of time. The authors express their appreciation to Wessex Water for its contribution to this article



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