ANITA™ range – Sustainable and cost-effective solutions for sidestream NH4-rich effluents

Lemaire, R.1, Chauzy, J.1, Christensson, M.2 and Bigot, B.3, 1Veolia Water, France, 2AnoxKaldnes AB, Sweden, 3Veolia Water Solutions & Technologies, UK



Anaerobic sludge digestion and all its optimised variants are key processes in the quest of self-sufficient or even energy-positive WWTP. However these processes lead to the generation of N-rich sidestreams which could reduce the energy benefit and the overall WWTP sustainability if they are treated on the mainstream due to increasing aeration and carbon substrate requirements. Therefore, energy- and cost-effective biological treatment dedicated to these sidestreams is recommended. In this paper, we are presenting two novel sidestream processes: A 1-stage deammonification MBBR process called ANITA™ Mox and an SBR process called ANITA™ Shunt for N-removal via the nitrite pathway. The first ANITA™ Mox full-scale plant was recently started-up in Sweden to treat around 200kgN/d (second plant under commissioning). It quickly achieved NH4 removal rate of 1.2kgN/m3.d with approximately 90% NH4 removal efficiency and does not require any pre-treatment, addition of chemicals or heating system. A 0.5m3 ANITA™ Shunt Pilot treating digested sludge filtrate achieved more than 90% N-removal via the nitrite pathway using on-line NH4 and NO2 measurement to optimise the SBR cycle in real-time and improve the performance of the process. The use of real-time calculated FNA level to control the duration of the aeration phases was very successful to reduce the N2O emission of ANITA™ Shunt process to very low level making it a very attractive and sustainable solution amongst other existing nitrite pathway processes. The first full-scale ANITA™ Shunt SBR plant treating 160kgN/d of reject water is now under commissioning.

Key words

Anammox, ANITA™ Mox, ANITA™ Shunt, MBBR, N2O emission, Nitrite pathway, Nitrogen removal, SBR, Sidestream treatment


Energy consumption is the main contributor of the total operation costs of a wastewater treatment plant (WWTP), thus energy savings is one of the priorities of each WWTP operator. In order to improve the energy efficiency of a WWTP, two actions have to be undertaken in parallel: “consuming less” and “producing more“. The organic matter contained in wastewater is a very important and valuable energy source. In the quest of self-sufficient or even energy-positive WWTP, anaerobic digestion of primary and secondary sludge is a key element. It is therefore important to separate a maximum of particulate organic matter from incoming raw wastewater, which can then be partially transformed into biogas through anaerobic sludge digestion and finally converted into energy by the use of combined heat and power (CHP) units. Every action taken towards improving the anaerobic digestion performances such as sludge thickening and pre-treatment by mechanical disintegration or thermal hydrolysis, better digester operation (mixing and continuous feeding), co-digestion of biowastes and other co-substrates will therefore bring the WWTP a step closer to self-sufficiency (Chudoba et al., 2010).

However, such improvements on the anaerobic transformation of organic matter from primary and secondary sludge to biogas often results in higher nitrogen level in the sidestream reject water that is recycled to the inlet of the WWTP and can constitute up to 20-30% of the overall incoming N load. In order to address both WWTP treatment efficiency (i.e. quality of treated effluent) and energy savings target, this increased nitrogen flux has to be removed without further increasing the energy consumption of the plant. Costly extension of the aeration capacity and of the size of aerobic/anoxic tanks together with increased consumption of costly external carbon source are also to be avoided.

Therefore, dedicated sidestreams treatment can be seen as a real long-term solution especially when considering energy- and cost-effective alternative to conventional N-removal via nitrification and denitrification processes (NH4+ à NO3 à N2) that is commonly used in WWTPs. N-removal via the nitrite pathway (NH4+ à NO2 à N2) and autotrophic N-removal (i.e. anammox-type processes) offer many advantages compare to conventional nitrification/denitrification processes such as (i) 25% reduction of O2 demand and 40% reduction in COD requirement for nitrite pathway processes and (ii) 60% reduction of O2 demand, no COD requirement and lower sludge production for autotrophic N-removal systems.

Nitrite pathway is achieved by creating conditions under which nitrite oxidising bacteria (NOB) are eliminated from the system, or at least inhibited, while ammonia oxidising bacteria (AOB) are retained or favoured by the operating conditions. Such conditions are (i) high temperature (i.e. > 25°C) as AOB have a higher maximum growth rate than NOB at such temperature (i.e. µmax,AOB > µmax,NOB), (ii) low dissolved oxygen (DO) condition due to the fact that AOB have higher oxygen affinity than NOB (i.e. KO2,AOB < KO2,NOB) and (iii) high levels of Free Ammonia (FA) and Free Nitrous Acid (FNA) which inhibit more NOB than AOB (Peng and Zhu 2006).

Sequenced Batch Reactors (SBR) have been widely used to achieve the nitrite pathway (Peng et al. 2004; Fux et al. 2006; Lemaire et al. 2008). It is well known that the intermittent feeding regime feature in SBR operation generates relatively high concentration gradient in the reactor. The periodic spike of FA after each feeding period creates a recurrent selective pressure against NOB. Once the nitrite pathway starts to set-in with some accumulation of nitrite during the aerobic period, cyclic inhibition of NOB due to high FNA concentration also takes place. The other main advantage of SBR systems is that it offers a great deal of operational flexibility by easy adjustment of aerobic and anoxic periods through temporal control of aeration and filling in a cycle with no need for separate basins, recycling lines or clarifiers. Therefore on-line control systems are more efficient and easier to implement in SBR than in continuous flow systems when aiming to achieve the nitrite pathway.

Autotrophic N-removal systems rely on two simultaneous biological processes:

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