Which is a reason for recirculation?

It dilutes strong sewage, keeps the film wet at low flow and helps reduce clogging

It removes the need for a secondary clarifier

It lowers the contact between sewage and film

Which statement on loading is correct?

High rate filters have 10-40 m^3/m^2/day hydraulic loading and use recirculation; low rate 1-4 without

Low rate: 10-40 m^3/m^2/day; high rate: 1-4

Low rate uses recirculation R = 0.5-3

How does effluent compare between the two types?

Low rate gives 80-90% removal and well nitrified stable effluent; high rate is not fully nitrified

High rate gives better nitrified effluent

Low rate gives lower BOD removal (65-85%)

2069 Bhadra · 8 marks

Recirculation in trickling filters; low vs high rate

Short answer

Recirculation returns filter effluent to inlet to steady hydraulic load, dilute strong sewage, increase contact, prevent ponding and seed microbes. Low-rate filter: hydraulic loading 1-4 m³/m²/day, no recirculation, 80-90% BOD removal, larger area. High-rate: 10-40 m³/m²/day, recirculation R=0.5-3, 65-85% removal, smaller area.

Why is recirculation necessary in trickling filters? Compare the low rate and high rate trickling filters.

Need for recirculation

Recirculation means returning a part of the filter (or secondary clarifier) effluent to the filter inlet, so that the filter receives $Q(1+R)$ , where $R = Q_R/Q$ is the recirculation ratio. It is necessary because:

It keeps the hydraulic load and flow over the distributor steady, even at night and in low-flow periods, so the film does not dry out.
It dilutes strong sewage, reducing the organic load per pass and avoiding odours and overloading.
It increases the contact between sewage and biological film and so improves BOD removal (recirculation factor $F=\dfrac{1+R}{(1+0.1R)^2}$ in the NRC equation).
It flushes the film and controls excess growth, which reduces clogging (ponding) and fly breeding.
It seeds the incoming sewage with active micro-organisms and returns dissolved oxygen.
It makes high loading rates possible in a smaller volume.

Comparison of low rate and high rate trickling filters

Point	Low (standard) rate	High rate
Hydraulic loading	1-4 m³/m²/day	10-40 m³/m²/day
Organic loading	0.08-0.32 kg BOD/m³/day	0.32-1.0 kg BOD/m³/day
Recirculation	Not used	Used (R = 0.5-3)
Depth	1.8-3 m	1.8-2.5 m (up to 3 m)
BOD removal	80-90%	65-85%
Effluent	Well nitrified and stable	Not fully nitrified, less stable
Dosing	Intermittent	Continuous
Area needed	Large	Smaller
Fly / odour nuisance	More (ponding, Psychoda flies)	Less
Sloughing	Periodic, heavy	Continuous, light
Capital cost	Higher (per unit flow)	Lower

✓ Answer checked by Claude Opus 5.5 (high effort)

Standard rate trickling filter, 1.2 MLD, NRC · Sanitary Engineering, chapter 7 · asked 3×
Standard rate trickling filter, 3 MLD, 300 mg/l · Sanitary Engineering, chapter 7 · asked 3×
Standard rate trickling filter, 4 MLD, NRC · Sanitary Engineering, chapter 7
Recirculation ratio, filter volume 510 m³ · Sanitary Engineering, chapter 7
Two-stage trickling filter, 7665 m³/day · Sanitary Engineering, chapter 7
HR trickling filter, 5 MLD, RR 1.5 · Sanitary Engineering, chapter 7 · asked 2×
RC low and high pass filters · Basic Electronics Engineering, chapter 1
Ideal low pass and high pass filters · Signals and Systems, chapter 5
High level vs low level languages · Computer Programming, chapter 1 · asked 2×
Low vs high spin; VBT of [Ni(CO)₄] · Engineering Chemistry, chapter 6

All Sanitary Engineering chapter 7 questions →

Recirculation in trickling filters; low vs high rate

Need for recirculation

Comparison of low rate and high rate trickling filters

Related questions