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CIWEM Wastewater Treatment Intensification London, December 2018
Pete Pearce, Farmiloe Fisher Environment Ltd
Conventional settlement benchmarks
Lamella settlement
belt filtration
Dissolved air floatation
Comparisons and conclusions
Conventional clarification needs a lot of space
primary clarifiers
secondary clarifiers
storm tanks
Activated sludge aeration tanks
Conventional clarifiers sized on surface (plan) area How do we get around this?◦ Rectangular tanks ‘nest’ better but are unpopular as
activated sludge clarifiers, acceptable for primary
◦ Ballasted settlement increase sedimentation rate
◦ SBRs –use the aeration tank as a secondary clarifier
◦ MBRs use membrane separation in lace ofsettlement tanks
◦ Biological aerated filters - utilise filtration
◦ Lamella settlement tanks
◦ Belt filtration
◦ Dissolved Air Floatation
• 50-60% TSS removal – 60-80% with coagulant addition
• 30-50% BOD removal – 50-60% with coagulant addition
• Simple, robust – tolerant of a wide range of operation, will tolerate medium term desludging failure
• Low maintenance• can be a sludge store – shouldn’t
be but can be in an emergency• Easily recovered following
failure/breakdown• Provides load balancing• Most of these apply to secondary
clarifiers also
• large air/water interface for odour release
• difficult to enclose/cover
Primary tank balancing of inlet AmmN peaks, inlet or eg return of liquors
Similar for secondary tanks, important if biological process is plug flow with short HRT, BAFFs, MBBRs etc
Uses the same settlement principles as conventional sedimentation tanks but provides a much higher settlement area per m2 plan area (6-7 fold increase)
Can be configured as plates or hexagonal tubes inclined at 60o
Plates can be side entry or bottom entry, tubes are bottom entry only
Performance range is similar to conventional settlement up to 80% TSS removal and 70% BOD removal with good coagulant addition
Applicable to primary and secondary sedimentation
Lamella Settlement –Inclined plates or tubes
60o
Projected area of inclined plate.
Plates need to be inclined by 60 degrees to allow for desludging.
Feed either
from bottom
or side
Clarified effluent
Sludge
Plate spacing needs to allow counter current flow of sludge and settled sewage typically 80mm
settlement area intensification ~ 6x
Secondary treatmentPrimary
treatment
Reduced footprint - lamella primary settlement Allows full containment for odour control
Year % TSS removal % COD removal
Commissioning 84 73
+5 71 68
Good
Poor
and plenty in between
0-5 years -→
Experience is very mixed from ~ a dozen large installations
careful attention needs to be paid to flow andsolids distribution
some designs more vulnerable to poor preliminary treatment, especially grit and FOG removal
Chemical dosing needs to be designed properly with regard to mixing and flocculation
In situ air cleaning is very beneficial
poorly designed units are very high maintenance
Few UK examples
careful attention needs to be paid to flow and solids distribution
Lamellas are a clarification process the solids thickening capacity which is essential for ASP applications is very limited - applied MLSS restricted to <1200mg/l. This will limit application
Higher MLSS loadings can be accommodated at significantly downrated hydraulic loading but this compromises their footprint advantage
For non ASP application solids loading may be too low to allow to allow plates/tubes to desludge adequately
Mean 11 mg/l95%ile 29 mg/l
relatively new development, small number of UK installations, more on the continent and in Scandinavia
High rate belt filtration maintained with frequent belt cleaning either with hot water, detergent or both
Very low footprint – easily covered for odour treatment
Very short retention time, no load buffering and no sludge holding capacity
Performance - solids capture vs filtration rate depends on filter belt specification)
Produces thickened primary sludge 4-6% DS
Air knife for solids removal
Sludge hopper to auger/pump
Filtered solids mat develops to aid filtration
Belt speed increases with headloss/flow
High pressure belt wash (non continuous)
Low pressure detergent addition for daily/weekly clean
Filtrate discharge
Variable water level
conceptually simple but engineering is more complex
Operational results are comparable to mid range municipal PST performance, TSS 40 – 55% removal , BOD 30-40% removal
Finer weave belts can be used for lower filtration rates with higher capture rates
belt solids loading is key performance parameter for the normal range of sewage TSS concentrations
Primary sludge produced directly as thickened sludge, Options for direct dewatering
Unlikely to be applicable for secondary application
• Relatively complex operation –flow/level/belt speed PLC controlled
• Very rapid response to changes in inlet condition or to component failure due to very low HRT
• large amount ancillary equipment required , blowers, hot water systems, detergent make up
• Requires continuous desludging, sludge breakdown will force unit shut down
• belt life is finite routine service item
in use for primary and secondary applications for many years at small number of sites (not UK)
New options (second generation) far more energy effluent than traditional due to optimised air saturation and micro bubble formation
More compact than lamella settlement, less than belt filtration
Very high removal efficiencies possible with coagulant addition, good achievable removals without
Produces thickened sludge (4-8% DS) directly
Easily covered for odour control
screened sewage
Compressed air 3-4 bar
saturator
Effluent recycle 15-50% at 3-4 bar
Bottom scraper for periodic use
effluent
Sludge float scraper
Sludge hopper
Introduction of supersaturated recycle stream
Floatation tank
modern DAF operates at lower recycle flows and pressures and so energy input is similar to pumping the effluent against a low head of 2-3m. Energy requirements 10-20wh/m3
Application BOD removal TSS removal FOG removal Sludge DS
Primary no coag. 50% 75% 60%* 6%
Primary + coag 70% 80% NM 6%
Secondary, ASP no coag
nd >99% Effluent <20mg/l
NA 2.0%
Dose response curve with iron dosing
short HRT so fairly rapid response to inlet condition or component failure
easily automated but sufficiently flexible to run manually. Nozzle/air injectors cleaned automatically
sludge DS controlled by surface scraper speed.
no sludge ‘storage’ so sludge removal needs to be continuous and reliable
reasonable variation in flow per unit tolerated, should not need to cascade units
conventional Lamella Belt filtration
DAF
Footprint* 100% 20% <10% 15%
Operability - complexity ++ + -- - Operability – flow range per unit
++ ++ -- +
Operability – emergency sludge storage
++ + -- -
Performance % TSS removal no coag
50-60% 50-60% 40-50% 65-75%
Performance % TSS removal + coag
70-80% 70-80% NA 80-90%
Recovery Very easy Can be complex
Can be complex
easy
Load balancing ++ + -- - Containment for odour treatment
-- + +++ ++
Suitability for secondary yes limited no yes
* based on process design only, excluding ancillaries, access
Options are available to reduce both primary and secondary settlement footprint
Lamellas can work very well for primary treatment but have a mixed track record – Need careful and expert design and are limited in scope for secondary treatment
Modern DAF looks very promising now that power requirements are reduced
Footprint is a trade off with complexity and with load buffering which needs consideration for UK style consents
Don’t overlook the hidden bonuses of conventional settlement – Be careful what you wish for.