9th European Waste Water Management Conference 12-13 October 2015, Manchester, UK SOLIDS REMOVAL SYSTEMS IN WASTEWATER TREATNMENT – TIME FOR SOME AGILE THINKING Bullen, C. and Matthews, R., Siltbuster Process Solutions UK Corresponding Author Email. [email protected] Abstract Solids removal systems are an essential part of any wastewater process and the use of modular technology allows for greater flexibility in providing cost effective solutions. The innovative approach that is desired by the UK water industry doesn’t have to be driven by new technologies or processes, but can simply be achieved by thinking differently with perceived non-standard solutions. Opportunities therefore arise for using familiar technologies in more strategic and smarter ways to provide robustness and reliability to existing treatment process. Dissolved Air Flotation (DAF) systems have readily been viewed as providing solids removal on clean water applications in the UK water industry. However, with the experience gained from the use of the packaged DAF treatment systems from other sectors, the process can fulfil a range of solids separation duties. This paper discusses the merits of adopting a modular approach to wastewater treatment to ensure existing assets can be fully optimised with a particular focus on the use of Dissolved Air Flotation technology as a high rate solids removal system. Keywords Dissolved Air Flotation, modular packaged treatment, process resilience, High Rate Solids Removal, TSS removal, COD removal, Introduction This paper will discuss the applications for off the shelf solutions and the benefits of thinking on a modular basis. As such, this approach offers major benefits which will resonate in the climate of AMP6; by enabling companies to get more out of existing assets and provide more treatment for reduced costs. In order to achieve this, there needs to be greater opportunity for off-site fabrication to become more widely accepted which also has the added value of construction efficiencies. A ‘bolt on’ modular approach often requires less design input due to off the shelf solutions which are typically above ground installations. This can result in reduced groundworks and associated risks within the project and potentially greater savings to the delivery programme. By focusing on the effectiveness of solids separation systems within the treatment cycle, it is possible to consider the robustness of the entire process to accommodate load demands, seasonal fluctuations or tightening consent conditions. Whilst there are times when adopting a conventional treatment approach is more appropriate it is worth taking a wider view on solids separation systems used in other applications and widely accepted across other industries and in the water industry outside the UK. In particular, there are opportunities for using DAF technology in a range of applications to maximise treatment capacity and resilience based on experiences gained by operating within industrial and municipal sectors. It presents the prospect for the UK water industry to learn from the wider www.ewwmconference.com Organised by Aqua Enviro Limited 9th European Waste Water Management Conference 12-13 October 2015, Manchester, UK applications, and to think about a more agile approach through adopting existing technologies in alternative operating environments. Adopting a Modular Approach Seeking efficiency through modular construction in the UK building industry is increasingly prevalent; be it to minimise site programmes or to ensure enhanced quality control through off-site manufacture. But how does the UK water industry approach modular construction? The use of some biological treatment technologies have provided some progress in modular approach, along with sectional concrete construction techniques for larger installations. However, there are wider opportunities for solids separation systems, particularly when looking to maximise treatment capacities through the entire process. One of the key challenges for the use of modular treatment systems is an acceptance of the need to align project objectives through the design horizons and design lives of the treatment process. For example, the construction of concrete structures is typically based on 80year design whilst the fit-out with steelwork and mechanical plant is typically 20year. Whilst whole life costings can take into account the repeat of mechanical refurbishment, it is questionable as to how effective this model is to maximising the asset life. By treating the solution as a complete package with a 20year (or more) design life, the whole life cost can be more readily aligned to the flow and load design horizons that invariably drive the sizing of new process equipment. Furthermore, the modular package concept allows for more cells / modules to be added to accommodate changes in peak flows / loads, either as a short term measure or where a longer term planning timeframe applies. Package treatment systems which use an off the shelf design often have to focus on maximising the treatment capacity within a small footprint, which can lead to some increased OPEX - but it is essential to understand the wider savings and benefits. The use of package treatment systems offers the versatility for the solution to be mobilised within short lead-in times and often minimal site preparation and importantly the opportunity to explore the option of creating semi-permanent facilities for peak load scenarios similar to those experienced within the industrial sectors. Figure 1: Lamella Clarifier Modular Installation1 The traditional approach to solids separation systems in the UK water industry is the use of radial or upward flow settlement tanks. Lamella systems have generally been accepted and used in clean water treatment scenarios but are gaining further exposure in wastewater scenarios. This can provide an increase in treatment capacity within a smaller footprint; however it is the use of DAF technology that provides a high rate (or intensive) solids removal system. The use of this modular packaged system can be demonstrated through our experience across the municipal and industrial sectors. www.ewwmconference.com Organised by Aqua Enviro Limited 9th European Waste Water Management Conference 12-13 October 2015, Manchester, UK Novel uses of Dissolved Air Flotation As outlined above, the standard solution for solids separation from bulk liquids in wastewater treatment is via use of settlement tanks (e.g. clarification of primary or humus solids and final settlement of activated sludge) or via use of filtration processes (e.g. micro screens or sand filters) for tertiary solids removal. Though DAF has been used in the clean water sector for over 40 years, it is seldom used in municipal wastewater treatment in the UK, even though additional treatment benefits can be gained from using the process. Presented below is a summary of the DAF process and projects we have undertaken on UK municipal wastewater works using modular packaged DAF systems and results from a series of test work carried. The DAF Treatment Process DAFs can be used for a number of treatment applications including: clarification (removal of suspended solids from bulk liquids), production of treated water with low Total Suspended Solids (removal of fines and colloidal material from bulk liquids) and for the thickening of sludge (as in the thickening of activated sludge). The DAF type, set up and operation will be dependent on the treatment requirements and the application to be used. For instance when used for solids removal from bulk liquids, the clarification area required will be key (as in humus solids removal) whilst in other processes (such as the thickening of activated sludge), the thickening area is key. Typically solids/liquid separation of effluent with a TSS concentration >3,000 mg/l is considered thickening and is driven by mass flux calculations as in final settlement tanks. The thickening area is determined by the footprint of the treatment equipment, not the effective separation area as provided by DAFs with lamella plates. Figure 2 presents a typical DAF configuration (i.e. a pressure DAF with saturator system). In a DAF air is introduced into the return water by means of a specifically designed system (Ref 4). The water/air mix is then pressurised (saturating the water with air) prior to the DAF whitewater being released into the inlet contact zone by means of pressure-reducing valve. The releasing of the pressure causes the air to come out of saturation resulting in bubbles being formed and rising to the surface of the liquid (with the operating pressure controlling the bubble size and hence the bubble rise rate, i.e. the higher the back pressure the smaller the bubble the smaller the rise rate). Suspended solids or particulates in the feed water attach to the air bubbles causing them to rise to the surface of the DAF, where they thicken by natural de-watering and are then removed by means of a mechanical scraper. Due to the generation of bubbles in the process DAF plants are ideally suited for the removal of near neutral buoyancy or poor settling solids. www.ewwmconference.com Organised by Aqua Enviro Limited 9th European Waste Water Management Conference 12-13 October 2015, Manchester, UK Figure 2: Typical Modular Lamella DAF The DAF treatment process can be enhanced by: altering and/or increasing the air flow rate in the whitewater system, use of lamella plates in the DAF unit, see Figure 2. Depending on the lamella plate spacing