In this article we will discuss about the design of trickling filters.
Organic Loading Rate:
Organic loading rate and the recirculation ratio are the main considerations in the design, where the organic loading rate is decided; the required filter volume is calculated. Now the depth and surface area of the filter are suitably chosen to secure hydraulic loading rates within the prescribed limits.
A number of equations are available for determining the efficiency of the plant, based on the organic loading rates and recirculation ratios. Rankine’s equation and the formula developed by the National Research Council of USA are commonly used in the design of trickling filters.
(i) Rankine’s Formula:
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For Single Stage Filters, the Ten State Standards states that the B.O.D. of the influent to the filter (including recirculation) shall not exceed three times the B.O.D. of the required settled effluent.
According to Rankine’s Equation:
Where S2 = B.O.D. for influent sewage after settling (mg/l)
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S4 = B.O.D. of filter effluent (mg/l)
R1 = recirculation ratio and
And E2 = efficiency of the filter.
The above equations are applicable only when the organic loading rate on the filter including recirculation is less than 1800 g/d/m3 and 10-30 m3/d/m2 hydraulic loading is maintained.
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When the organic loading ranges between 1800-2800 g/d/m2 following equation is used:
For all the loadings in excess of 2800 g/d/m3, the B.O.D. removal is assumed to be 1800 g/d/m3 only.
When the effluent of the first stage filter is applied to second stage filter without settling, following equations are applied.
In case the first stage effluent consists of settled sewage, which does not pass through the first stage filter, following equation is used,
The value of recirculation is given by,
Where Q1= total flow through the first stage filter
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Q = raw sewage flow.
The B.O.D. loading on the filter is determined by the equation,
Where, La1 = BOD- loading in kg/day
Q = sewage flow in mLd.
The efficiency of B.O.D. removal for the second stage filter is less than the first stage, because the amenability to treatment of the applied B.O.D. is affected by previous treatment. Therefore for the second stage filters, the Ten State Standards stipulate that the B.O.D. of the sewage applied to the second stage filter including recirculation shall not exceed two times the B.O.D. expected.
Following Rankine’s formulae are used in this case:
Where S6 = B.O.D. of settled effluent from second stage (mg/l),
R2 = recirculation ratio is second stage,
E3 = efficiency of second stage filter.
(ii) NRC Equations:
These are the empirical expressions developed from a study of the operation results of trickling filters serving military installations in U.S.A. These equations are applicable to both low rate and high rate filters.
The efficiency of single stage of two stage filter is given by
For the second stage trickling filter, the efficiency
Where E2, = percentage efficiency in B.O.D. removal of the single stage or first stage of the two stage filter.
E3, = percentage efficiency of second stage filter.
W1 = B.O.D. loading of settled raw sewage in the single stage/first stage of the two stage trickling filter in kg/day.
W2 = W1 (1 – E2) = B.O.D. loading on second stage filter in kg/day.
V1, = volume of first filter in m3.
V2 = volume of second stage filter in m3.
F1, = recirculation factor or number of effective passes for first stage filter
R1 = recirculation ratio for the factor stage filter.
f = treatability factor (e.g. for sewage).
F1 = recirculation factor or number of effective passes for second stage filter.
R1, = recirculation ratio for the factor stage filter.
f = treatability factor (e.g. for sewage).
F2 = recirculation factor or number of effective passes for second stage filter.
R1 = recirculation ratio for the second stage trickling filter.
F = treatability factor (e.g. for sewage)
F2 = recirculation factor or number of effective passes for second stage filter.
R2 = recirculation ratio for the second stage trickling filter
Eckenfelder Equation:
Where L6 = BOD of unsettled filter effluent in mg/l.
La = influent B.O.D. including recirculation in mg/l.
D = depth of filter bed in m.
Q = How in mLd.
A = area in hectares.
Galler and Gotaas Equation:
Galler and Gotaas, based on a multiple regression analysis of data from existing plants and providing for the effects of recirculation, hydraulic loading, filter depth and temperature of the sewage, developed the following equation.
L6 = unsettled filter effluent B.O.D., mg/l
Li = filter effluent B.O.D., mg/l
D = filter depth, m.
i = influent flow, mLd.
r = recirculation flow, mLd.
a = filter radius, m.
T = sewage temperature, °C.
Galler and Gotaas suggested that recirculation improves the performance of the filter but established that a ratio of 4: 1 was the practical upper limit for recirculation.
... Total B.O.D. applied to the trickling filter
= 230.4 × 5.3 × 106/1000 gm/day = 12,21,120 gm/day = 12,21,120 gm/day
As the B.O.D. in the effluent is desired
= 50 gm/lilre
... Efficiency of the filter unit = 230.4 – 50/23.0 × 100 = 78.3%
The efficiency of a filter trickling from formula (16.16)
Where E = efficiency = 78.3
W = weight in gms/day of B.O.D. applied to filter = 12,21,120 gm/day.
V = Volume of the filter in cu.m.
F = Number of effective passages of the sewage thorough filter
From Eq.(16.17)
Recirculation:
In high rate trickling filter the effluent is again sprinkled over the filter media, which is known as recirculation.
Fig. 16.6 illustrates the line diagrams of some of the common methods of recirculation of sewage effluent.
The following are the advantages of recirculation:
(i) The period for which the filter remains out of order is reduced to a minimum by adjusting the recirculation to influent flow.
(ii) The thickness of organism film is decreased by forced film sloughing.
(iii) The raw sewage is freshened and foul odour is prevented from it.
(iv) The applied sewage is seeded with active organisms and enzymes of effluent, due to which the efficiency of filter is increased.
(v) The lower portion of the filter media becomes more effective, increasing the overall efficiency.
Recirculation Factor:
Recirculation ratio is the ratio of recirculated flow to the total flow of raw sewage. The capacity of the pumps doing the recirculation of sewage is determined with this. The increased load on the trickling filters is also determined by this.
Recirculating pumps capacity = (Influent Sewage Flow) × (Recirculation Ratio)
Hydraulic load on Trickling filters = (Influent Sewage) × (1 + Recirculation Ratio)
The hydraulic recirculation factor
= (1+ Recirculation Ratio).
If the sewage has high concentration of B.O.D., it can be treated by passing number of times through the trickling filters and settling tanks. But practically, it has been noted that there is not much use in recirculating the sewage for more than 2-3 times. This is due to the less response of the organic material (to be removed) to removal and it goes on reducing in each successive recirculation cycle.
The common recirculation ratios are given in table 16.1.
Example:
Design a high rate trickling filter plant to treat settled domestic sewage having BOD of200 mg/l for an average flow of 22.50 mLd to satisfy an effluent BODs of 10 mg/l. Adopt peak factor as 2.25.
Solution:
As the efficient BOD5 required is less than 30 mg/l, a two-stage filtration plant shall be provided. While designing the plant, the filters are designed for average flow only. The distribution arms, under drainage system and other pipe lines etc. shall be designed for peak flow and checked for average flow.
The hydraulic loadings for different R1 values in terms of kLd/m2 for the average flow is circulated and given in the table below:
