Water and Effluent Treatment Magazine
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Caption if needed sdkvjb sdkvjnsdlk dvsdvdvsdvsdvsdv dv skjbdv lskdjvbsdkljvbsdkv processes were taken off-line for refurbishment. DAF units can be located between existing primary tanks and biological treatment pro- cesses to provide enhanced pri- mary solids separation and to remove the high levels of BOD associated with the solids. This has the effect of reducing the BOD load onto the existing bio- logical processes and provides the overloaded process with a chance to operate within its original design parameters. The advantage here is that there is no 'seeding time' required, as with biological systems, so if the cause of a potential consent failure is high BOD loads, an immediate solution can be put in place, without the time required for a rented SAF unit (Submerged Aerated Filters) or MBBR (Moving Bed Biofilm Reactor) to seed. DAF processes can be used downstream of aerated biologi- cal treatment units, such as SAFs or MBBRs to provide sec- ondary solids separation prior to discharge. The flocs gener- ated by aerated biological pro- cesses are o†en very fine and can be poorly settling. This makes DAF units ideal for this type of application, as the pro- cess relies on flotation. SPS recently supplied Thames Water with a whole mobile treatment process (See picture) consisting of primary solids removal, via lamella clar- ifier, and secondary treatment, comprising of a Siltbuster MBBR unit for biological treat- ment and a Siltbuster DAF for secondary solids separation. This system was used during planned maintenance work on the site's activated sludge plant. During this refurbishment work, there was a need to ensure that the client's dis- charge consents were main- tained. The results showed a 75% removal of solids without the use of chemical addition, to comply with the required 30 mg/l consent standard (Tab 2). SPS is currently undertaking an extensive programme of work providing DAFs for ter- tiary solids separation. Hire equipment has been provided to various Southern Water sites, where a number of reed beds are being refurbished. These existing reed beds provide ter- tiary treatment downstream of Humus settlement tanks and are being taken out of service during the refurbishment work. DAF processes are ideal for this type of application, as the efflu- ent from Humus tanks is o†en difficult to settle, and the small, near colloidal particles, o†en make physical processes such as drum filters ineffective. This is particularly the case in some 'P' removal schemes where chemicals are dosed. This is because the chemicals blind the screen cloth and increase the backwash flows through the plant. This can further overload existing humus tanks. The addition of the lamella plates within the DAF process increases the effective separa- tion area of the unit, making the required footprint typically 10 percent of a conventional DAF. This significantly increases the hydraulic capabilities of the unit, particularly on low strength wastes below 1000 mg/l. For example, a 40 litre per second tertiary solids separa- tion would require just one DAF unit, with a 15 m2 footprint. So, wherever the problems are on a sewage treatment works, a DAF process will mitigate these risks effectively and with a smaller footprint than traditional solutions. ONSITE SEWAGE TREATMENT 14 WET NEWS JULY 2018 | wwtonline.co.uk TABLE 3 – TYPICAL RESULTS OF TSS CONCENTRATIONS FROM HUMUS TANK AND POST DAF Post HST Post DAF Removal Rate TSS (mg/l) TSS (mg/l) % Max 29 10 79.3 Average 27 8 68.7 Min 25 6 60.0 95 Percentile 29 10 78.0 MBBR 30 biological process (right) with Secondary Solids Separation (left) via DAF