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.