SOLIDS REMOVAL SYSTEMS IN WASTEWATER TREATMENT TIME FOR SOME AGILE THINKING

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1 SOLIDS REMOVAL SYSTEMS IN WASTEWATER TREATMENT TIME FOR SOME AGILE THINKING Dr Chris Bullen, Technical Manager & Rich Matthews, General Manager Siltbuster Process Solutions 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, 1.0 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 applications, and to think about a more agile approach through adopting existing technologies in alternative operating environments. 2.0 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. -1-

2 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 Installation 1 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. 3.0 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. 3.1 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. -2-

3 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. The DAF treatment process can be enhanced by: Figure 2: Typical Modular Lamella DAF 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 the footprint can be reduced by approximately 90% of a conventional DAF resulting in a compact unit with an intensive treatment capacity. Use of chemical pre-treatment using coagulants and/or flocculants can increase particle sizes and hence further intensify the process. However chemicals are not always necessary, see below. 3.2 Municipal DAF Applications As a result of a number of factors, (for example ferric dosing for phosphorus removal, use of plastic media, poorly aerated biological filters, tightened discharged consents, growth in the catchments), a number of small to medium municipal wastewater treatment works across the UK have been experiencing difficulties with managing the risk of discharge consent failures. The threat to water companies serviceability performance figures at the time of the AMP 6 final determination (end of AMP 5), led to a high demand for mitigation equipment to ensure discharge consents were being met (particularly for Total Suspended Solids (TSS), Total Iron and Total Phosphorus (Total P) compliance). Issues associated with standard treatment methods such as micro screens (high backwash water volume, solids particle size too small) or moving bed sand filters (lead time too long) led to an alternative treatment method being sought. As the characteristics of the solids that required removing were similar to those of raw water in low land clean water treatment plant (where the solids can be due to algae, colour or near colloidal particulates) and to those of industrial cooling water applications (which DAF is used as standard treatment solution) DAF was proposed to the municipal wastewater sector. Following on site test work, which confirmed the process was suitable (the small air bubbles were able to lift the very near colloidal particles without the need for chemical addition), full scale trials were undertaken which again showed suitability of the process. Subsequently a number for full -3-

4 TSS Concentration (mg/l) scale projects have been undertaken. The results from 8 such projects are summarised below, where there was a requirement to ensure either TSS, Total Iron or Total P limits were met Tertiary DAF Applications Figure 3 below presents the average feed and treated water TSS results from 8 projects undertaken using the DAF process as tertiary solids removal (treating flow rates from 7 l/s to 45 l/s by packaged modular DAF units). The DAF performance (and hence treated water quality) is dependent on a number of factors including: biological process where the solids were generated, solids loading on to the DAF, air to solids ratio, strength of solid floc to be removed. In all cases there was a requirement to install the mitigation equipment in a limited time frame (ideal for packaged modular processes) whilst access and/or land availability was limited in a number of the projects. All projects were successful with operating timescales ranging from 2 to 14 months. With several of the projects, the DAFs were installed as semi-permanent solutions whilst long term remediation solutions could be confirmed. The projects also showed that, contrary to common perception, the required treated water quality could be met without the need for chemicals to be used. This was an important factor, since the use of chemicals was a concern to the operators due to increased operational costs and the risk of an uncontrolled release of chemicals to the water course Inlet TSS Outlet TSS Site Figure 3: Tertiary Treatment DAF Projects Average Treated Water TSS Results Without Chemicals Figure 4 presents photographs of typical tertiary DAF applications. Figure 4: Typical Tertiary DAF Projects Activated Sludge DAF Separation Applications During AMP6 there will be focus on undertaking more maintenance led projects and getting more out of existing assets. But how do you take the final settlement tank in a single stream activated sludge plant off line? As DAF is a more intensive process than settlement (and can thicken activated sludge to in excess of 3% solids), mobile modular DAF packages can offer a solution. -4-

5 TSS Concentration (mg/l) Figure 5 presents average TSS results for 3 recent activated sludge separation projects. These were undertaken on sites that either required the Final Settlement Tank (FST) to be taken off line for maintenance, where the Return Activated Sludge (RAS) system was not functioning adequately or for thickening activated sludge to reduce the site Mixed Liquors Suspended Solids (MLSS). This resulted in the need for a solution to be installed quickly and to produce a treated water with low TSS concentrations. Figure 5 shows that even when the DAF feed was in excess of 5,000mg/l the treated water TSS concentration was reduced to below 27 mg/l without the use of any chemicals. Figure 6 presents a photograph of a DAF being used for activated sludge solids liquid separation. 10,000 Inlet TSS Outlet TSS 1, Site Figure 5: Activated Sludge DAF Projects Average TSS Results Figure 6: DAF used as final activated sludge solids liquid separation 3.3 Further DAF Treatment Applications Through projects undertaken in our industrial division, we have seen that DAF can be used for many applications on a municipal wastewater plant. Further work undertaken has shown that due to the DAF s intensive nature and small footprint (particularly when used with lamella plates) DAF can provide a solution to non-standard solutions raised due to issues such as limited land availability, tightened discharge limits and growth in the catchment. A series of DAF tests have been undertaken to evaluate the possible use of DAF as primary and humus solids removal as well as to confirm the tertiary solids application performance. These were undertaken on municipal wastewater from a biological filter works to compare the following treatment processes: Settlement only (No chemical addition) DAF with air only (No chemical addition) Settlement with coagulant added (as used in Total P removal schemes) DAF with coagulant added (as used in Total P removal schemes) Tertiary DAF Tests Initial tertiary treatment tests (Settlement and DAF) were undertaken to confirm the performance of DAF as a tertiary treatment method without the use of chemicals. A second set of tests were undertaken to review how the addition of ferric chloride (as used for Total Phosphorus removal) affects both settlement and the DAF process, see Figure 7. The results showed: DAF with air only (no chemical addition) showed an enhanced TSS water quality (and hence removal rate) compared to the use of settlement and can produce a treated water quality similar to that produced by moving bed sand filters. The addition of a coagulant (as used in Total P Removal schemes) did not enhance the treated water TSS removal rate, but provided added benefit of lowering the treated water Total P (a treated water Total P of 0.5mg/l was produced). -5-

6 Treated Water TSS (mg/l) COD Removed (kg/day) Treated Water TSS (mg/l) It is noted that the use of a coagulant (ferric) in tertiary solids settlement, see Figure 7 Settlement with Coag result, increased the TSS concentration hence requires careful control as can lead to the formation of fine, near colloidal poor settling solids (iron hydroxide particles) Without chemicals With chemicals Figure 7: Tertiary DAF Tests Average Treated Water TSS Results Primary solids separation DAF Tests A further application for DAF is as primary solids removal. Though this has been tested on the continent (Ref 5) and is used frequently as preliminary/primary treatment on industrial treatment applications, DAF has not been used as a primary solids removal system in the UK. Results presented in Figure 8 show that using DAF as primary treatment could improve the TSS removal rates as follows: DAF (without chemical addition) enhances primary solids removal by 58% as compared to removing TSS by settlement, Using chemicals (a coagulant) in the DAF generates a treated water of less than 20mg/l TSS Without chemicals With chemicals 0 Without chemicals With chemicals Figure 8: Primary DAF Tests Average Treated Water TSS Results Figure 9: Primary Tests Treated Water COD Loads Removed The enhanced TSS removal would have the additional benefit of lowering the organic load on to the downstream biological plant as follows (see Figure 9): The treated water COD load reduced by a further 37% by using DAF as compared to settlement The addition of the coagulant (ferric) in the DAF treatment stage reduced the DAF treated water COD to just over 100mg/l (just under 80% removal) Use of DAF with a coagulant increased the COD removal by 160% as compared to settlement with no chemicals Humus solids separation DAF Tests As with single stream activated sludge plants, the refurbishment of single stream biological filter plants can be problematic and to avoid the installation of a new humus settlement stage, mobile packaged settlement tanks (e.g. mobile lamella clarifiers) are used. If space is limited DAF can also offer a solution as higher treatment capacity per m 2 of footprint can be provided. -6-

7 Treated Water TSS (mg/l) Treated Water TP (mg/l) Results presented in Figure 10 and show that: Using DAF without chemicals as a humus solids liquid separation can produce a treated water TSS of less than 15mg/l (compared to settlement quality of 50mg/l) and Using DAF with chemicals (through the addition ferric) reduced the treated water TSS to 7mg/l. Due to the improved TSS removal rates achieved by the DAF as compared to settlement additional particulate P is also removed, see Figure 11. This results in DAF treated water having a 45% reduction in Total P as compared to settlement. This indicates that DAF can provide a viable option for achieving Total P consent compliance. The use of Ferric further reduces the Total P concentration by removing dissolved P Without chemicals With chemicals Without chemicals With chemicals Figure 10: Humus DAF Tests Average Treated Water TSS Results Figure 11: Humus DAF Tests Average Treated Water Total P Results 4.0 Investment and Operational Considerations As previously discussed, the principle of adopting modular or packaged treatment solutions offers the opportunity for reduced site activities, leading to reduced risk from either operational interfaces or construction methods. However, the key principle is that by adopting a modular outlook, the design life can be aligned to the future loads demands. Whilst the principles outlined for these types of intensive solids separation systems do attract a significant CAPEX saving, there will be a marginal OPEX offset. However the AMP6 environment now exists to challenge the OPEX and CAPEX balance and allows for a more agile approach for treatment solutions using off the shelf products with a proven performance. As a simple scenario, the construction of a typical 7.5m diameter conical settlement tank (20l/s FFT) is tabulated below in comparison to a lamella clarifier system to reflect some of the construction and operational considerations. -7-

8 Item Number of process units Conventional 7.5m dia Radial Flow Clarifier 1No Process reliance on single process unit built to convention design standards and construction techniques Typical Operating Area / Plate area 44m 2 100m 2 Asset Life Material of construction Installation Capital costs Ground / temporary works Operational Considerations Tank - 80year M&E Plant 20yr Typically concrete construction 210k direct costs *. Typically below ground structures requiring significant investigations and temporary works Conventional and familiar technology with minimal interface other than scum and sludge removal system Other Traditional design and construction techniques deployed refined to company specific design standards. *Direct costs are for the construction and installation therefore exclude the design and other non-contract costs Table 1 Comparison of Conventional Clarifier to Modular Lamella Clarifier System Modular Lamella Clarifier 2No Configuration for redundancy during low flows or maintenance requirements either future expansion with additional units Effective plate area Tank 30yr M&E Plant 20yr Typically mild steel with painted systems or stainless dependant on application 130k direct costs* Typically above ground installation requiring ground bearing slab Intensive process with use of lamella plates requires periodic draindown and plate cleaning. Twin unit arrangement - no comprise to treatment during maintenance activities Minimal design with offsite construction approach with factory testing to reduce site construction and commissioning activities Some significant differences are listed in Table 1 should a non-standard, modular solution be sought compared to a conventional treatment system. The site specifics and project drivers will dictate whether the opportunities arise for such non-standard solutions to be considered as option for solids separation. The conventional approach should always remain as part of the suite of options, but giving due consideration to proven technologies from other industries allows the UK water industry to seek cost effective solutions. 5.0 Conclusion This paper has sought to demonstrate that opportunities exist for the UK water industry to think more broadly in accepting technologies and approaches that are proven in a wider application from other sectors. The benefits of modular, packaged systems not only facilitate the off-site construction approach but also seek to ensure a robust solution is offered for the process with a particular emphasis on solids separation. The use of a lamella DAF has been shown to offer a high rate or intensive solids separation system, serving as an example of proven existing technology being deployed into wider non-standard applications. Through the examples of site installations for tertiary treatment, the package DAF system has provided some robustness to existing processes with minimal interference. The test work has demonstrated that further potential exists by using the lamella DAF in a more intensive scenario of primary settlement, achieving a high rate removal which brings greater capacity into the system and reduced load to downstream processes. Adopting a modular approach offers the opportunities to enhance existing asset performance with minimal interference with existing operations whilst ensuring it is robust and accommodates future demands. The use of this technology is more widely accepted in Europe and other industrial wastewater sectors and should therefore provide a strong foundation for wider applications to be considered in the UK water industry. References 1. Cuckfield WwTW, Southern Water Permanent HST Installation with lamella clarifiers 2. Dissolved Air Flotation As Superior Pre-Treatment for Municipal Wastewater Treatment 3. UKWIR Report Ref No15/WW/04/18 Optimising the Balance Between Primary and Secondary Treatment 4. Dissolved Air Flotation for water clarification: Edzwald and Haarhoff (2012) 5. Dissolved air flotation as superior pre-treatment for municipal wastewater treatment: Menkveld HWH, Broeders E (2014) -8-