Peter Dold, Bill Alexander*, Mark Fairlamb, Gillian Burger, Daniela Conidi, Chris Bye and Weiwei Du EnviroSim Associates Ltd., Hamilton ON Canada; * Alexander Process Consulting, Sandton, South Africa

MODELLING FULL-SCALE GRANULAR SLUDGE SEQUENCING TANK PLANT PERFORMANCE

GRANULAR SLUDGE SEQUENCING TANK (GSST) OPERATION IN

BioWin

MIXING PHASE

• Granules and mixed liquor well mixed.

• Aeration continuous or on/off.

• DO setpoint or air flowrate.

SETTLING PHASE

• Granules immediately settled on base of the reactor (with a void volume).

• Mixed liquor solids settle on top of the granular sludge.

• Waste activated sludge (WAS) withdrawn from bottom layer of settled mixed liquor solids.

• Granules not removed during wasting - only bulk mixed liquor.

FEED PHASE

• Influent feed typically commences well into the settling period.

• Upper section of the reactor should be well-clarified liquid.

• Distributed across the base - into granular sludge voidage.

• Plug-flow up through the reactor.

• Displaced liquid overflows into launders as effluent.

DECANT PHASE

• Small decant of clarified liquid to drop the liquid level below the launders.

• Avoid spillage of mixed reactor contents when next cycle starts.

MODELLING APPROACH

• Granular sludge based on BioWin’s 1D biofilm model. Biofilm thickness equivalent to granule radius.

• Settling of mixed liquor (non-granule) solids based on 1D flux model. Liquid above bed of settled granules divided into n equal-depth layers during settling.

• Full BioWin ASDM applied throughout.

• Detailed phys-chem modelling (pH, chemical precipitation, gas/liquid mass transfer, etc.).

….. CONSEQUENCES AND LIMITATIONS

• Does not predict new granule “formation”.

• Diameter of granules changes dynamically depending on loading, and solids impingement/erosion.

• Granule size distribution not considered.

• No steady state solution. GSST model simulated from initial values for approximately 4 SRTs to reach quasi-steady-state.

Parameter Unit Modelled Measured

Net reactor solids concentration kg m-3 10.3 > 8

Percent total mass in the granular phase % 81 > 80

Sludge production kg d-1 4,050 3,900

Effluent

Concentration

TSS mgSS/L 18 20

TCOD mgCOD/L 68 64

BOD5 mg/L 4.2 9.7

TP mgP/L 0.9 0.9

Soluble PO4-P mgP/L 0.4 0.4

TN mgN/L 7.1 6.9

Ammonia mgN/L 0.8 1.1

MODELLING A FULL-SCALE SYSTEM

BioWin GSST model calibrated to the full-scale Nereda® process at Garmerwolde, Netherlands*:

• Buffer tank followed by two parallel GSSTs with a total volume of 19,200 m3 (9,600 m3 each).

• Average dry weather influent flow is 28,600 m3/d.

*Pronk, M., de Kreuk, M.K., de Bruin, B., Kamminga, P., Kleerebezem, R. and M.C.M van Loosdrecht. (2015), Full scale performance of the aerobic granular sludge process for sewage treatment. Wat. Res., 84, 207-217.

TYPICAL SIMULATION RESULTS

User specifies:

• Initial granule settled volume fraction (FG).

• Initial granule diameter (D).

• Voidage fraction (E) in settled granule bed.

• Operating cycle.

TYPICAL RANGES:

• D from 0.6 to 1.5 mm.

• E from 20 to 28%.

• Granular surface area to tank volume ratio [A/Vt] from 300 to 500 m2/m3.

Dynamic behaviour in the system is quite complex:

• BioWin GSST model captures ability of these systems to remove N and P.

• Growth (and decay) of different biomass species within the granule and bulk mixed liquor.

User-defined parameters used to calculate a base granular surface area (A) [in metric units] :

Modelling results demonstrate capacity of the GSST to biologically remove nitrogen and phosphorous.