The most innovative use of existing RBC technology.
5 April 2011, News release from KEE Process Limited
The Rotating Biological Contactor technology has been around for a long time and is commonly known as RBC. Traditionally RBCs have been used in small scale rural wastewater treatment applications in packaged plant configuration to provide secondary treatment prior to discharge of the treated wastewater into a watercourse.
For some years, RBCs have been accepted by Water Utility Companies for treatment of medium sized flows for less onerous discharge consents. For tighter consents they have traditionally used activated sludge process and possibly membrane reactors (MBR). These technologies are high in capital cost, high in energy demand and require extensive operational input. Increasingly Process Engineers are now looking for technologies which provide a reliable solution and low lifetime cost, with simple operation, maintenance and servicing requirements.
For their site at Derrylin, Northern Ireland Water (NIW) had consent from the Northern Ireland Environment Agency to achieve a final effluent with BOD<5mg/l, Suspended Solids<5mg/l, NH4-N<5mg/l and 6< pH < 9 on a 95%ile basis.
NIW Project Managers and Process Engineers were tasked to find alternatives to using MBR technology as required by their process matrix selection. Having worked with KEE Process as their RBC Framework Partners for Western & Southern Division the Project Team asked KEE Process Ltd to produce a design that would be capable of consistently providing the consented effluent standard.
It was also necessary to design the system for flows and loads for the 2010 horizon and ensure that this could be further expanded to account for almost 50% increase in flows and loads for the 2030 horizon whilst maintaining the performance objectives.
KEE Process applied their well tried and tested Managed Flow Fixed Film RBC technology to provide NIW with a solution that offered simplicity, robustness and low soluble BOD and NH4-N concentration. To ensure that the process has low energy demand and is simple to operate, the team decided to use slow sand gravity tertiary filters to reduce the solids in the final effluent.
The brief to KEE was to provide design options for achieving the specified final effluent and address the Client's concerns with regard to conventional technologies on offer.
The required benefits were:
- Simplicity in design and operation
- Reduced capital expenditure
- Minimal on-site construction tasks to save time and costs
- Reduced power consumption to lower carbon footprint
- Lower operating and maintenance labour costs
- Lower on-going spares and replacement parts costs
- Robust and consistent performance to maintain the treatment objectives for discharge of the treated wastewater to a watercourse
- Mitigation of the effects of extreme weather conditions on the process stability of the plant
- Capital equipment design life longevity and assurances
- Reduction or elimination of noise and smell nuisance
- Minimal visual impact on the surroundings
Effective and reliable process design and innovative application of prefabricated GRP settlement tanks; RBC fixed film technology housed in Factory built GRP enclosures with sectional insulated covers and the option of heating the space under the covers, enabled the above concerns of the Client to be addressed. RBC technology is simple in operation and has extremely low power consumption. The fixed film process is self-regulating and therefore requires minimal day-to-day operational labour input.
Benefits achieved were:
- specification ensured 30 years+ design life for the components and reduced planned preventative maintenance requirements to bi-annual lubrication change;
- structural components have 20 years warranty; this together with lowpower requirements and labour demand enabled the client to minimise the 20 years lifetime costs;
- prefabricated factory built equipment meant that the site work was minimal and the CAPAX costs were low;
- insulated covers on the RBC biological stage with the optional undercover heating mitigated the effect of extreme weather conditions on the process stability; covers also eliminated the noise and smell nuisance;
- substantial underground installation of the equipment with covered RBCs minimised the visual impact on the local surrounding environment;
- Process control is further aided by the innovative 'Flow Control Mechanism' fitted to each RBC.
There are a number of innovative concepts included in this project to ensure process robustness and minimal operational costs.
The RBCs are fitted with managed flow technology to enable "plug" flow conditions in the biological stage and to unify the wastewater character to be treated downstream in the subsequent RBC stages. The innovation also enables the process to cope with hydraulic and organic peak loads and to maintain process stability when concentration of inhibitory substances exceeds threshold values. This innovation has been in commercial use for 16 years, although until now, the technology had not been used to achieve the high degree of treatment required at Derrylin.
With the high degree of treatment, oxidation of all NH4-N results in loss of alkalinity. To recover alkalinity and maintain the effluent within consent with respect to pH, KEE introduced the second innovation of RBC anoxic stage for denitrification. The first stage RBC was designed to be oxygen limited and included internal recycle.
For the process to operate satisfactorily at all times with respect to nitrification, it is necessary to maintain a minimum temperature of 6 °C in the biomass. The biological process is exothermic and is normally self-sustaining with respect to heat. However, when the ambient temperatures are sub zero it becomes necessary to provide supplemental heat to the process. KEE have achieved this by controlling the space temperature under the covers around the aerobic biological process by building in the option to introduce warm air into the space for the nitrification process to remain at its optimum.
These simple innovations have made it possible to achieve the desired objectives of maintaining the simplicity whilst keeping costs low as the space heating system only operates during extreme low temperature conditions.
The KEE RBC plant with tertiary slow gravity sand filter provided the clients with more than 30% saving in capital and construction costs. Some of the cost benefits are associated with prefabricated equipment housed in structural, corrosion-proof lightweight GRP housing designed for underground installation and fitted with insulated lightweight GRP covers, whereas some of the CAPAX cost savings relate to simplicity and inherent robustness of the equipment.
However, lower CAPAX and OPEX costs savings are in themselves not enough to embrace the technology if it fails to deliver the functional objectives documented previously. The plant supplied by KEE for Derrylin has delivered consistent plant performance, low lifetime costs, low carbon footprint and substantially lower capital cost. Compared to other nearest compatible process technology, the 20 year lifetime power saving with using KEE RBC plant with tertiary slow gravity sand filter is £317,185, maintenance and operational labour, sludge disposal costs, and spares costs saving is £272,420; making a total 20 years Operational lifetime saving of £589,605.
There are further latent cost savings, which were not included by the client in the costing, but can now be capitalised as follows:
1. In case of power failure at site, the plant includes enough buffering and balancing and therefore discharge from the plant would be maintained within consent for at least 6 hours. Due to low energy demands of the process, a standby generator can be easily moved onto site within the 6 hour period.
2. In case of mechanical breakdown, there is ample redundancy available in the equipment arrangement and therefore there will be no need to cater for a temporary plant during repair and maintenance.
The performance results for Derrylin plant are documented below:
NOTE: * Maximum value represents 100%ile compliance with respect to effluent discharge.
Design final effluent: BOD5(ATU)<5mg/l, SS<5mg/l, NH4-N<5mg/l & 6< pH < 9
With the benefit and experience of the above performance results, low lifetime costs, low carbon footprint and substantially lower capital cost, the Client has endorsed this technology into their selection matrix. The Derrylin project team has also put together a business case for using the same simple RBC concept and process design approach for a much larger municipal wastewater treatment plant (3 times larger than the Derrylin plant) to deliver similar final effluent concentration with respect to BOD and SS, but with much tighter consent for NH4-N at 1mg/l, all on a 95%ile compliance basis. For this larger plant, compared to other process technologies, the 20 year lifetime power saving with RBC is £947,200, Maintenance & Operational labour, sludge disposal, and spares saving of £813,520 making a total 20 year Operational lifetime saving of £1,760,720.
In the UK, it is now common for the environmental Regulators to issue tighter consent for discharge, particularly when the receiving waters are sensitive or are of special scientific interest or have shellfish and do not have adequate dilution to reduce the impact of treated wastewater discharge. Technical solutions are already on offer to the Water Companies or Developer, or for sites with no mains drainage, to treat wastewater to a high standard but these solutions are complex, expensive and require much more energy to operate. KEE's solution to use a simple to operate fixed film RBC technology coupled with a simple low energy tertiary filter gives the end users choice of simple technology with low carbon footprint and less operational input.
To enable the application of the simple and low energy RBC technology for treating small flows to a high standard KEE have also developed a simple physical biological filter that is available as an integral unit in a small packaged plant or as a stand-alone prefabricated modular system to follow a medium sized modular plant configuration to provide tertiary treatment for high quality discharge. The technology has also been adopted for use in framework for small-flow packaged plants with built-in tertiary filter and internal recycle to achieve high quality final effluent for discharge to sensitive receiving water.
In areas where clean water is a precious commodity, the technology offers a simple low energy solution for the treated wastewater to be re-used for irrigation or non-portable uses such as toilet flushing, general outdoor washing etc. In the Caribbean, Middle East and Africa, KEE have already supplied package systems to treat small flows where the plants also include UV disinfection system after the tertiary filter to enable safe water re-use.
The most salient features are simplicity of the process plant operation and low energy demand. KEE tertiary filter is simple and includes aerobic and anoxic stages for treatment of filtered fine complex and high molecular weight organic particles. This negates the need to use high-pressure back wash pumps with blowers for scouring the filter medium.