Things can only get better so it’s said…… For the UK water industry this has meant the need to produce better effluent quality, with an expectation of lower customer prices. New means of carrying out treatment works evaluations and optioneering improvements have the potential for step changes in making choices for the long term benefit of our environment within acceptable cost limitations. WRc is shaping the future in this area by collaborating with other innovative organisations to provide opportunities to link skills of scientific evaluation and modelling to those of practical engineering design and construction.
The UK water industry is rising to both environmental and cost challenges. Submissions made by the utilities in England and Wales to OFWAT for AMP7 recognise their role in meeting the challenges in the UK Government’s 25 year Environmental Plan. Specifically the Water Industry National Environmental Plan (WINEP) describes a range of measures to achieve particular objectives by 2025. The expectation is that investments must be sustainable to achieve long term environmental improvements, enabling natural resilience for the future.
The WINEP database shows many proposed changes to discharge consents for all wastewater businesses. The most significant are to phosphorus consents, but also include changes to ammonia, BOD, and other determinands for large and small works. Many are for new or significantly more stringent discharge quality consents.
For the wastewater businesses key drivers continue to be compliance, efficiency and the most effective use of current infrastructure and WRc has always been at the forefront of this thinking. Challenges include growing populations and public interest in minimising environmental impacts and health impacts and social impacts. Much of the potential for improvement stems from incremental changes to existing operations and sites due to substantial existing structures and location requirements…. Water only gravitates downhill! But changes can be made through better understanding current performances and the potential for better monitoring and control, on-site, remotely and on-line.
The WINEP database shows 908 phosphorus schemes, with requirements to achieve a range of discharge consents concentration values for phosphorus down to 0.25mgP / l (and a few even lower). A range of solutions including new installations (and potentially new technologies) as well as supplementation of existing systems will be required. As part of the UK Chemical Investigation Programme (CIP) a variety of treatment technologies have been trialled by the EA / UKWIR. Most of the tested processes use tertiary attachment technologies – reed beds with a steel slag aggregate, ferric salt treated effluent applied to sand filters (Dynasand filters), reactive filtration and adsorption (BluePro), coagulation using magnetite and settlement separation (CoMag Evoqua), and enhancing EBPR. The range demonstrates something of the choices that operators will need to consider to achieve new discharge compliance limits.
Associated effluent discharge limits for metals can add to the compliance challenge, which may be breached with, for example, poor settlement or tertiary filtration. A good understanding of the most risky parts at a treatment works enables choices to be made in operating the plant, use of new monitoring technologies, or changes and supplementation to existing processes.
Trials carried out by WRc on mixing effectiveness showed that it is possible to use process modelling to test the efficiency of an existing dosing system against optimum performance targets, thereby identifying if there is scope for improved dosing. Experience in the USA is that low levels of phosphorus can be achieved with relatively low doses of iron, providing that there is good initial mixing between the wastewater and the iron. New metals are being proposed to precipitate phosphorus, which offer the scope of reducing the molar dose, reducing the sludge volume, and having better performance across a wider range of alkalinity.
The process modelling approach can be used much more widely for overall treatment efficiency improvements. Many wastewater treatment works across the UK are currently operating outside of their design capacity / capability and as a consequence present a significant compliance risk. The cost of replacement or upgrade of these assets using traditional approaches to mitigate this risk may place an excessive demand on capex funding and/or timing of investment.
Options to retrofit into, or supplement existing processes, are now routinely considered. For example, for activated sludge processes the IFAS process is seeing greater interest. The MBBR variant was evaluated over 20 years in the UK, but then relegated to a few uses as a roughing process ahead of conventional activated sludge. Now IFAS is being considered for sites where the aim is to increase the biomass contribution without increasing the MLSS or needing major expansion of settling tank capabilities.
Bioagumentation is another old process that is returning. In the 1980s / 1990s it was proposed as a means of allowing over-loaded nitrifying plants to add more bacteria to continue to meet the ammonia removal requirements. However, the costs of adding this biomass supplement made the process less economically attractive. A decade ago WRc looked at using means of running ‘breeder’ systems to create the supply of nitrifiers onsite, but did not find a configuration that succeeded. But continued work by others has paid off. Greener Waste Technology
are offering a new form of breeder reactor (Biocube) that uses a small tank to produce nitrifiers to bioaugment the main activated sludge units. WRc is working with several water companies and the supplier to evaluate if the new systems have overcome the problems of the past.
Whilst new processes are attractive, making use of new or improved understanding, and replacing old less efficient equipment with new equipment with long life and efficient processes, it is also possible to improve existing processes through performance optimisation. This is where systematic optioneering and comparisons of efficiency with benchmarks becomes a route to choices between evolutionary improvement of performances or replacement or addition of processes. Retrofit technologies and low build / no build solutions have the capability to enhance & extend performance and can offset capex and compliance risk.
Online measurement is increasing capable of being integrated with process models, either as “black-box” systems using AI learning to use inputs and outputs measurements and formulate best condition solutions, or process models that include a variety of levels of direct or empirical physico-chemical and biological growth and conversion relationships to produce process outputs from which adjustments to process conditions can be made to assess potential improvement or maintenance of set target compliance conditions.
As an example, Murphy Process Engineering, supported by WRc and others has been developing an investigation and evaluation system for existing WwTws. This uses an automated sampling analysis scheme that can be installed at a WwTW and integrating this with a process model of the treatment works based on the WRc STOAT dynamic sewage treatment model. Sampling is carried out continuously using a sample loop demonstrated to accurately reflect the source water, and delivered to instruments set up in a self-contained, fully autonomous package container. WRc has supported Murphy in assessing the scheme and modifying STOAT to exchange data directly with SCADA systems.
This system is envisaged as being capable of deployment for a period of weeks at a site in order to develop base data on a full range of quality determinands. It can be used to take samples from influent, inter-stage process streams or final effluent, and ideally would be used to carry out an intensive survey of a treatment plant. Instead of a few daily composites there are measurements at multiple locations, with a sampling frequency of 30 minutes or better. At the same time as the survey is establishing data that can be used as a basis for comparing performance against various metrics the data is also used to evaluate the performance of closed loop online control. Murphy is using the Hach controllers for aeration and chemical demand control, so that the results will permit an economic evaluation of the benefits of implementing advanced control for the tested sites.
For looking at the wider catchment alternative approaches are needed, such as WRc’s SIMPOL catchment model which permits the inclusion of sewers, CSOs, rivers and sewage works. SIMPOL is currently being used to assess the effects that catchment growth will have on sewage works and the sewer network, and to identify where upgrades are required.
The vision 40 and more years ago that it would be possible to predictively model wastewater treatment and impacts on a catchment is steadily becoming more accurately and readily achieved with benefits for our current and future environment, whilst not entailing excessive costs for improvements to wastewater treatment installations.
By Richard Addison
Head of Treatment Processes