Technically speaking

This article is a summary of the invited lecture to the CIWEM Seminar in Leeds on June 8 this year, given by Doctor James Barnard, senior process director at Black & Veatch, on Biological Nutrient Removal (BNR) – Past Present and Future. BNR is celebrating its 30th birthday this year as the first findings on biological phosphorus and nitrogen removal were published in 1974 and patented by the National Institute for Water Research (NIWR) in South Africa under the name Bardenpho (Barnard denitrification and phosphorus removal). Biological nutrient removal (BNR) has come of age and is now applied around the world.

In Canada and the US, the pressure for phosphorus removal at first centered on the Great Lakes and estuaries on the east coast where eutrophication became a serious problem. That effort has since expanded to nitrogen removal for a large number of estuaries and lakes. Effluent requirements vary but tend to get more stringent, especially with the introduction of total mass daily loads (TMDL) in the US, which set the permissible effluent mass discharge for a specific plant to remain the same in future when the plant expands.

Initial standards for phosphorus removal required removal to below 1mg/l. The present trend seems to aim for 0.3mg/l or even lower. Lack of space and higher standards have focussed treatment efforts on more compact plants with combinations of biological and chemical treatment in both suspended and attached growth processes. Achieving phosphorus concentration of less than 0.01 and nitrogen concentration of less than 1mg/l is presently under discussion. The high cost of removal to such low levels makes a better evaluation of the effect of different nitrogen and phosphorus species on the environment necessary.

NITROGEN REMOVAL

Towards the end of the 1960s eutrophication emerged as a serious problem in many parts of the world. Studies showed that while phosphorus was the growth-limiting nutrient for algae in lakes and reservoirs, nitrogen was limiting in the marine environment. The first efforts at full-scale biological denitrification focussed on suspended growth systems and the three-sludge system was proposed with one complete activated sludge stage for carbon removal, another for nitrification, and a third for denitrification with methanol addition. These systems incurred the high cost of multiple clarifiers and an added carbon source, since the organic carbon in the plant influent is destroyed in the first aeration stage of the process.

The introduction of channel-type activated sludge systems, especially in the Netherlands and in South Africa, showed the occurrence of simultaneous nitrification and denitrification with total nitrogen removal of up to 85% in extended aeration plants. Barnard developed the two-stage denitrification/nitrification system (later referred to as the modified Lutzack-Ettinger or MLE process) for nitrogen removal up to 85% and the four-stage Bardenpho process for nitrogen removal in excess of 90%.

Many alternatives such as on/off aeration, sequencing batch reactors, intermittent aeration, alternative aeration (bio-denitro) and simultaneous nitrification and denitrification processes have been proposed. In order to compete with larger plants, primary settling was added and the SRT reduced to as little as six-eight days. The difficulty to control oxygen input, bulking and the instability of the channel systems required that anoxic zones be added turning channel-type plants into quasi-MLE plants, which seemed to combine some simultaneous nitrification and denitrification (SND) of the channel systems with better use of the influent COD for denitrification in a separate zone, leading to lower TN levels.

fighting the big tank syndrome

Limited space has spawned other innovations for more compact systems such as attached growth systems, step-feed nitrification and denitrification, combinations of suspended and attached growth systems and membrane bio-reactors (MBR). The aim for combining suspended and attached growth systems is to achieve nitrification and maximise denitrification using internal carbon sources in suspended growth systems such as the MLE configuration, followed by attached growth denitrification such as denitrifying sand filters to achieve very low levels of total nitrogen (TN).

Step-feed nitrification and denitrification was introduced in the UK, France and Germany in the late 1970s in order to allow storage of more solids in smaller basins and avoid the large recirculation of several times the flow necessary for typical MLE configurations. This approach was further developed by the city of New York, which had a total of 14 high-rate step-feed plants but was required by the Long Island Sound Initiative (LISI) to reduce nitrogen discharge first to 50%, then to 75%, of the load that was discharged before sludge dumping at sea was banned from the plants discharging to the LISI. The step-feed nitrification/denitrification was implemented for nitrogen removal at 13 plants. Each plant consisted of a four-pass system with a third of the primary effluent passing to the top end of pass numbers two, three and four. Pass one was used for return activated sludge (RAS) re-aeration. High-rate sequencing batch reactors (SBR) have no specific advantages over conventional systems for nitrogen removal where space is available. At best, 85% TN removal is possible. They lend themselves to fitting into limited space and can be stacked as was done for Dublin in Ireland. A view of the tanks is show in Figure 3. Increased demands for nitrogen removal in previously high-rate plants without enlarging the footprint has led to the development of hybrid processes.

Plastic media is added to the aeration basins to increase biomass without additional solids load on the clarifiers. The media supports a higher biomass in addition to the mixed liquor, which allows nitrification at much shorter solids retention times than for a standard BNR plant. Mixed liquor can be recycled to the pre-anoxic zone for denitrification in admixture with feed.

Fixed media such as woven strings or fabrics have problems with heterotrophs over-growth and worms infestation. With floating plastic media such as extruded rings or floating sponges, there is continuous friction that control excess slime growth on the surface. The mobile media is retained by screens and cannot be washed out. An example of an IFAS system used in the MLE configuration is shown in Figure 4.

The results of a pilot study with mobile media at the Broomfield plant in Colorado showed that at a temperature of 14°C, it was possible to get nitrification with a 4.5day SRT, which is half of what is expected. Good simultaneous nitrification and denitrification was also observed leading to more than 85% removal of total nitrogen. Pilot plant results are shown in Figure 5.

From bnr to BPR

The theory of biological phosphorus removal is well-established, requiring an initial anaerobic zone free of nitrates to contact phosphate accumulating organisms (PAO) with volatile fatty acids (VFA). Since nitrogen removal to low levels are not always required and some nitrates may be present in the return activated sludge (RAS), many variations were proposed to reduce the nitrates in the RAS before returning it to the anaerobic zone. The Westbank process shown in Figure 6 is a successful configuration that combines all the features of the above flow sheets to safeguard the anaerobic zone against nitrates and dissolved oxygen the return sludge. The RAS is passed through the pre-anoxic zone for further denitrification and 5-10% of the influent may be added to the RAS. The high-mixed liquor concentration encourages endogenous denitrification, which is augmented with some carbon from the influent to reduce the nitrates. The main influent stream is passed to the anaerobic zone to which fermenter supernatant is added. Storm flow is diverted to the first anoxic zone to protect the anaerobic zone. Mixed liquor from the nitrification zone is recycled to the first anoxic zone for denitrification.

Building on the theory originated by Fuhs & Chen, the need for a source of volatile fatty acids (VFA) in the influent was recognised. In warmer climates with flat sewer grades and a number of forced mains from pumping stations, VFA will be produced in the solids deposited in the sewers and on the slime layers in forced mains. However, it was clear that in colder countries and with combined sewer systems some augmentation of VFA would be required. Fermentation of primary sludge was first used in Kelowna, British Columbia, Canada.

Follow-up studies showed that fermentation of primary sludge was essential to the operation. With combined sewer systems, fermentation is essential due to the low strength and low temperature during winter rainfall events.

New challenges

Phosphorus can be removed by chemicals, but as the desired concentration goes down, the consumption of chemicals goes up due to unwanted side reactions. Pre-treatment by biological means can reduce the ortho-phosphorus to less than 0.1mg/l with the addition of fatty acids or generation of these acids on-site. Less chemicals would then be required to reduce the effluent phosphorus to less than 0.03mg/l, as has been demonstrated in various plants using Bardenpho, three-stage Phoredox or MUCT pre-treatment with post-chemical addition and effluent filtration.

It has been demonstrated in a small full-scale plant in New York that effluent phosphorus well below 0.01mg/l was possible with the use of membranes. Configurations for getting both low nitrogen and phosphorus are being investigated and will be compiled in a report sponsored by the Water Environment Research Foundation (WERF) about a year from now.