Where are we now and where are we going in meeting the challenges for sustainable ammonia treatment? While providing an overview of current technologies and best practice for ammonia removal, in both wastewater and sludge liquors, this article also considers the treatment works of the future, where the benefits of new treatment process configurations could be realised.
Ammonia removal from municipal wastewater has historically been achieved by the same mechanism; the use of naturally occurring bacteria to oxidise ammonia (nitrification) into nitrate and in some instances, subsequently reducing that oxidised compounds (denitrification) to inert nitrogen gas.
Whilst this is still the basis for ammonia removal in most wastewater applications, great progress has been made in how this is applied in order to meet increasingly stringent consent limits, whilst minimising both cost and environmental impact. In contrast to wastewater treatment, ammonia removal from sludge liquors has seen a greater level of innovation with some technology departing from the traditional biochemical pathways.
The EU Water Framework Directive is driving more stringent discharge consents for both ammonia and phosphorus to meet environmental quality standards (EQS). The ammonia consent standard will largely determine what technologies may be employed, the level of process control/automation required and the associated energy/carbon impacts of achieving this standard.
Assessing and identifying the most cost effective and sustainable solutions for ammonia compliance is not always straight forward. This could involve improved process operation / control, expansion of existing capacity through conventional or novel technology, treatment of sludge liquors to reduce ammonia load or a combination of these approaches.
Ammonia removal – the basics
The industry standard approach to ammonia treatment is to employ naturally occurring micro-organisms to provide biological oxidation (nitrification) to convert ammonia to nitrate (via nitrite). Where a total nitrogen consent exists, or it is deemed beneficial to the treatment process, biological reduction of the nitrate (denitrification) is used to convert this to inert nitrogen gas. The former of these reactions requires a significant input of oxygen and requires alkalinity, whilst the latter of these will return both oxygen and alkalinity into the process but requires a readily available carbon source.
The design considerations for ammonia removal are very well understood and discussed extensively in the literature (hence the equations are not reproduced here), but to give a very brief summary, compliance is typically a function of a) maintaining a sufficient population of bacteria to treat the applied load, and b) ensuring the metabolism of these bacteria is not rate limited (ie have sufficient oxygen and alkalinity). The slower growth rates of nitrifying bacteria in comparison to those carrying out the carbonaceous treatment (removal of organics) means that significantly more treatment capacity must be provided to achieve effective ammonia removal compared to BOD removal only.
Most biological nitrification processes require mechanical aeration for the provision of oxygen. As nitrification requires around 4.5 kg O2 per kg ammonia removed in comparison to just 1kg O2 per kg BOD removed (ie four and a half times more oxygen), the energy impact of tighter consents can be very high. Aeration is typically quoted as being responsible for 60-70% of the total energy consumption on a WwTWs and so energy efficiency is usually a key consideration in process selection.
Options for Process Improvement
- Improved Process Control
Optimising assets to eliminate/reduce the need for capital investment to meet tighter consents is always desirable. Trickling filters are a very low energy and sustainable process capable of nitrification, but these are often replaced with more energy intensive systems due to these being more reliable for low ammonia compliance. There may well be instances however where, with the correct process modelling and optimisation, these processes could be compliant. Research has shown that well-maintained and optimised trickling filters can reliably meet a 2 mg/l ammonia consent (Pearce et al).
Activated sludge plants are often up-rated with more aeration capacity and improved automation. Installing real-time control can improve energy performance by operating close to but within consent. This ensures compliance whilst avoiding the cost and carbon impact of over-treatment. This also provides the capability to enable innovative cost/energy saving practices such as variable consenting and demand side management, the latter of which involves shutting down part or all of the process during peak energy demand whilst ensuring compliance is not breached. Whilst it is not currently monitored, some work has shown that excessive reduction in aeration may influence the unintentional emissions of nitrous oxide from the process, but more research is needed to fully understand how this should influence process control philosophy.
- Additional treatment stages – preliminary / tertiary treatment
Where additional biological treatment capacity is required, it can be more cost effective to employ tertiary treatment technology for ammonia removal, rather than extend the secondary treatment capacity. There are a wide range of tertiary treatment technologies for ammonia removal including nitrifying trickling filters (NTFs), submerged aerated filters (SAFs) and biological aerated flooded filters (BAFFs) as well as solids removal technologies such as sand filters (ie aerated continually operated upflow filters COUFs) that can be configured to provide some ammonia removal.
Like all biological treatment processes, tertiary ammonia removal processes need sufficient feedstock to maintain an active biomass inventory. Over-performance of upstream processes can lead to reduced nitrifying biomass populations, which reduces the tertiary treatment plants capability to respond to peaks in ammonia load. This is most notable where a carbonaceous activated sludge plant may nitrify for a time and then during peak ammonia load, when there is break-through of ammonia the tertiary plant provides insufficient treatment.
Given these issues, it can sometimes be more cost effective and robust to install a pre-treatment process (such as a roughing filter) to shed some of the load in order for the existing secondary plant to become compliant, rather than to invest in tertiary treatment (Koodie et al).
- Process Intensification
Some of the most interesting advancements have been in the intensification of the secondary biological treatment process. These innovations typically either a) maintain a much higher biomass inventory within the existing plant size b) provide conditions that allow different bacterial reactions/kinetics to occur, or do both. Some of the most notable examples of process intensification technologies recently installed in the UK are summarised below:
PROCESS INTENSIFICATION TECHNOLOGIES
HYBACS – Modular integrated pre-treatment stage for activated sludge to increase overall plant capacity and performance with the pre-treatment influencing the activated sludge biomass
IFAS – fixed-film media installed within activated sludge aeration basin to increase biomass inventory and provide characteristics of both a suspended growth and attached growth process
MABR – membrane aerated bioreactor which provides attached growth within an activated sludge system as with the IFAS whilst providing low-energy aeration through the membranes
Nereda – Alternative to conventional activated sludge and often retro-fitted, to provide a granular aerobic sludge. The rapid settlement and high sludge density maintains a much higher biomass inventory whilst the anaerobic/anoxic/aerobic gradient through the granule provides enhanced biological nutrient removal (BNR) characteristics.
Further information can be obtained from suppliers.
Direct dosing or side-stream treatment with formulations of nitrifying bacteria is also sometimes advocated as a form of process enhancement. As these are the same bacteria that occur naturally, it would be desirable that these should develop through normal operation. However, these systems can offer advantages where challenges exist such as a large tourist population requires short-term response to ammonia peaks or intermittent toxicity from a trader requires a mechanism to quickly re-populate to maintain nitrification.
- Liquor treatment
On larger works where sludge treatment occurs and in particular where there is a large quantity of sludge imports, the ammonia load from the return liquors can be very significant (up to 30-40% of the ammonia load). It may therefore be more cost effective to invest in liquor treatment than to expand the existing wastewater treatment works. Additionally, the implementation of advanced anaerobic digestion processes has increased both the load and concentration of ammonia from sludge treatment.
Liquor treatment technology has seen a lot of innovation with some technologies using new ways to control the bacteria or in some cases completely new bacterial pathways.
The main technology types are summarised below:
- Conventional based processes – Sequencing batch reactors or the AMTREAT process rely on conventional processes but are adapted to the high temperatures and ammonia concentrations. Process control is typically simple with high levels of removal.
- Nitritation / denitritation processes – Careful control of the sludge age at higher temperatures can alter the bacterial populations to only partially oxidise the ammonia (to nitrite) before it is removed as nitrogen gas. This offers aeration and carbon savings but requires increased process control.
- Deammonification (Anammox processes) – This involves a completely new pathway performed by anammox bacteria (which form red granules) which react ammonia with nitrite offering greater aeration savings and no carbon requirement. Influent composition and process control are very important.
- Physical / chemical removal (ammonia stripping) – Ammonia is stripped and recovered as ammonium sulphate through a pH increase. This is not typically employed in municipal wastewater although some reference sites exist on food waste liquors. This is included as increases in liquor strength could influence its deployment in the future.
Predicting the future is never easy. For some time, the holy grail of wastewater treatment has been the concept of a low-temperature, full-flow anaerobic process which could be applied in colder climates such as the UK. Research has focused on anaerobic membrane bioreactor (anMBR) technology with a view to changing carbonaceous treatment from being energy intensive to a net energy producer. If full flow anMBR were to become economically viable, then this would produce a low BOD, zero-solids effluent containing both nitrogen (predominantly ammonia) and phosphorus.
This would favour N & P recovery, perhaps through secondary treatment using algae lagoons (as demonstrated in the EU All-gas project). Research by Cranfield University has shown how algae can be immobilised in gel beads to create a very low hydraulic retention time process for tertiary nutrient removal.
With a robust and well-operated anMBR, ion exchange processes could also become viable. Ion exchange in wastewater treatment is fraught with problems, particularly with bio-fouling and solids blinding, but the technology exists for both ammonia and phosphorus recovery. Ammonia can be removed and recovered as ammonium chloride using naturally occurring minerals called zeolites, whilst nano-technology media has been developed for the removal and recovery of phosphorus.
The TOTEX approach to investment in AMP6 and into AMP7 supports maximising existing assets to reduce CAPEX. It is therefore reasonable to expect increased use of advanced process control and process optimisation techniques as well as deployment of technologies that offer process intensification.
Low-cost remote monitoring and process optimisation to maintain low-energy solutions such as trickling filters for smaller works is also desirable.
The introduction of deregulation in the sludge market may introduce some uncertainty around the treatment and management of sludge liquors in the near future.
Process consultants Aqua Enviro are part of the SUEZ Group.