High-Rate Trickling Filters: Merits and Demerits | Waste Management

In this article we will discuss about:- 1. Features of High-Rate Trickling Filters 2. Comparison with Conventional or Standard Trickling Filter 3. Proprietary Types 4. Merits and Demerits.

Features of High-Rate Trickling Filters:

High-rate trickling filters are similar to conventional or standard trickling filters except that the rate of loading (both hydraulic as well as organic) for high rate filters is several times more than that for conventional trickling filters. The rate of loading is increased by adopting the process of recirculation of sewage in high-rate trickling filters.

It consists of pumping back a part of the filter effluent or final effluent (i.e., effluent from the secondary settling tank) to the filter unit. The addition of treated sewage effluent in raw sewage improves the quality of later and make it possible to increase the rate of loading.

A high-rate trickling filter is constructed in much the same manner as a conventional or standard trickling filter. Thus most of the constructional features of high-rate trickling filters are similar to those of conventional or standard trickling filters.

However, some of the distinguishing features of high-rate trickling filters are as indicated below:

1. Filter Depth:

The high-rate trickling filters are shallow in depth. The depth of filter media is reduced to about 0.9 to 2.5 m so as to obtain better aeration needed to achieve high rate of biological activity.

2. Filter Media:

High-rate filters employ rock, slag and synthetic materials such as plastic modules as filter media. However, super-rate filters or roughing filters employ synthetic materials such as plastic modules or modules consisting of redwood slats as filter media. The physical characteristics of commonly used filter media are given in Table 14.4.

3. Stone Media:

As indicated in Table 14.4 with increase in stone size the specific surface area decreases but the percent void space increases. The recent trend is towards the use of larger size stones especially for high-rate trickling filters.

The current specification for stone media is that when mechanically graded over vibratory screens:

100% should pass through 110 mm square mesh

95-100% should be retained on 75 mm square mesh

0.2% alone should pass through 50 mm square mesh

0.1% alone should pass through 25 mm square mesh

The above percentages are by weight.

4. Plastic Media:

Plastic media is one of the most common type of synthetic media used for high-rate and super-rate trickling filters. The plastic media have specific surface area, high void space and low weight. The plastic media consist of interlocking sheets of plastic which are arranged like a honeycomb to produce highly porous and clog-resistant media which permit higher hydraulic and organic loading rates.

The plastic sheets are corrugated so that when the medium is assembled, a strong lightweight grid is formed. Subassemblies of the medium form modules that can be arranged to fit any filter configuration. In trickling filters employing synthetic media, aerobic organisms (heterotrophic) are present throughout the depth of the filter media, primarily because of the higher oxygen transfer capability even with the higher hydraulic loadings.

Filters as deep as 12 m have been used with synthetic media. Synthetic filter media have of late been used successfully in super-rate been trickling filters for the treatment of strong industrial sewage or domestic sewage mixed with strong industrial sewage.

5. Single Stage and Two Stage Trickling Filter Plants:

High-rate trickling filter plants may be either single stage filter plants or two stage filter plants.

In a single stage filter plant sewage is passed through a single filter only and hence if there are two or more filters they are operated in parallel. Sewage is, however, recirculated to each single stage filter. Thus a single stage filter plant consists of a primary settling tank, filters operating in parallel, secondary settling tank and facilities for recirculation of the effluent.

The flow diagrams for single stage high-rate trickling filter plants are shown in Fig. 14.8 (a) which shows the various patterns of recirculation. Settling before recirculation may be carried out either in primary settling tank as shown in Fig. 14.8 (a) (i), or in secondary settling tank as shown in Fig. 14.8 (a) (ii). Recent studies have indicated that for municipal sewage direct recirculation without settling is as effective as recirculation of settled effluent and hence recirculation pattern shown in Fig. 14.8 (a) (Hi) may be adopted.

For single stage filters the depths of filter media usually range from 0.9 to 2.5 m with an optimum range of 1.5 to 2.0 m.

In a two stage filter plant sewage is passed successively through two filters provided in series. Thus a two stage filter plant consists of a primary settling tank, two filters in series, an intermediate settling tank which may be omitted in certain cases and a secondary settling tank.

Recirculation facilities are provided for each stage. The effluent from the first stage filter is applied on the second stage filter either after settlement (in the intermediate settling tank) or without settlement. Some of the common flow diagrams for two stage filter plants are shown in Fig. 14.8 (b). In Fig. 14.8 (b) (i) an intermediate settling tank or clarifier is used for settling the first stage effluent before it is applied to the second stage filter and the recirculation is only through the settling tanks.

In Fig. 14.8 (b) (ii) the intermediate settling is omitted and the recirculation flows are settled. In Fig. 14.8 (b) (iii) which is known as the series-parallel system, part of the settled raw sewage is applied directly to the second stage filter increasing the efficiency of that stage. In Fig. 14.8 (b) (iv) there is neither intermediate settling nor settling of filter effluent prior to recirculation.

For two stage filters the depths of filter media in each filter also usually range from 0.9 to 2.5 m with an optimum range of 1.5 to 2.0 m for the first stage filters and 1.0 to 2.0 m for the second stage filters.

Two stage filtration will provide a higher degree of treatment than the single stage filtration for the same total volume of filter media. Two stage filters are used for the treatment of strong sewage when the effluent BOD has to be less than 30 mg/l.

6. Recirculation:

Another special feature of a high-rate trickling filter is recirculation which is in contrast to a conventional or standard trickling filter. Recirculation consists of returning a part of the filter effluent or final effluent (i.e., effluent from the secondary settling tank) to the filter. Recirculation is necessary to provide uniform hydraulic loading as well as to dilute the high strength wastewaters. Recirculation is expressed in terms of recirculation ratio, R.

7. Recirculation Ratio:

The ratio of the recirculated flow to the sewage flow is known as recirculation ratio R. This ratio determines the required capacity of recirculating pumps and the hydraulic load on filters. Thus, we have-

Capacity of recirculating pump

= R x (influent sewage flow)

Hydraulic load on filter

= (1 + R) x (influent sewage flow)

Recirculation ratios usually range from 0.5 to 3 and values exceeding 3 are considered to be uneconomical in the case of domestic sewage but ratios of 8 and above have been used with industrial wastes.

8. Recirculation Factor:

The number of effective passes through the filter is known as recirculation factor F, and is given by the equation-

9. Multiple Units:

In a single stage filter plant, it is advisable to split the required filter volume into two or more units so that when one filter is taken out of operation for maintenance or repairs, the entire sewage can be passed through the remaining units, overloading them temporarily.

In a two stage filter plant if multiple units are proposed in each stage, the entire sewage may be routed through the remaining units of the stage when one filter in that stage is taken out of operation.

However, the recirculation flow is maintained at the original level operating the stage at a lower recirculation ratio. If, instead, only one filter is proposed for each stage a bypass should be provided for each stage. In the case of two stage filters the volume of filter media is usually divided equally between the first stage and the second stage units.

10. Plant Hydraulics:

The feed pipe to the filter, the distributor, the underdrains and the main collection channel should be designed for the peak instantaneous hydraulic loading on the filter. In low-rate filters the peak loading will be the peak discharging capacity of the dosing siphon or the dosing pump. In the case of high-rate filters, the peak loading on the filters will be the sum of the peak rate of sewage flow and the constant recirculation rate.

When multiple units are used for the high-rate filters in any stage, the hydraulics of the plant should be checked for peak loading with one filter out of operation, the entire flow routed through the remaining units. A reduced recirculation ratio is adopted for this condition so as to reduce the peak loading and avoid oversizing of the piping.

When multiple units are used care should be taken to ensure that the flow is divided between the various filters.

11. Pumping Arrangements:

In a high-rate filter, pumping is required for recirculation. Pumping may also be required for lifting the filter effluent to the settling tank or to the next stage filter.

Except in the case of small plants, recirculation pumps should be installed in multiple units so that the recirculation rate can be changed as found necessary.

Pumps for lifting the flow through sewage should have adequate capacity to pump the peak flows through the plant. The pumps should be installed in multiple units to take care of diurnal variations in flow which will approximately be the same as the sewage inflow to the plant. It is further necessary to provide storage in the suction well equal to about 10 minutes of discharge capacity of the lowest duty pump. Float control arrangements are desirable in the suction well for controlling the number of pumps in operation.

In all the cases, at least one pump should be provided extra as a stand by. Also, in the case of recirculation pumps, flow measuring and recording devices are desirable on the discharge line so that a record can be kept of the recirculation ratio actually employed in the plant.

Comparison with Conventional or Standard Trickling Filter:

To give a comparative idea of the working of conventional or standard trickling filters and high-rate trickling filters, the salient features of both of them are compared in Table 14.5.

Proprietary Types of High-Rate Trickling Filters:

There are following three principal proprietary types of high-rate trickling filters:

1. Biofilters (Dorr Co)

2. Accelo-filters (Infilco Inc.)

3. Aero-filters (Yeomans Bros.)

The patents for two of them cover the method of recirculation while the patent for the third covers the method of application of sewage to the filter. However, through cross-licencing and expiry of patents these features no longer control the use of high-rate trickling filters.

1. Biofilters:

These are relatively shallow filters with depth 1.2 to 1.5 m, and utilize recirculation of a portion of the filter effluent to the primary settling tank for a second passage through the filter.

Fig. 14.9 shows the flow diagrams of biofilter plants giving the following three types of treatments:

(a) Single stage intermediate treatment

(b) Single stage complete treatment

(c) Two stage complete treatment

In the case of industrial wastes which are strong two stage biofiltration may be used. In two stage biofiltration recirculation is used for each stage. The total filter volume of two stages is the same as a single stage. By appropriate selection of flow diagrams and recirculation ratios the desired degree of treatment can be achieved.

The organic loadings adopted normally range from 9000 to 11000 kg of 5-day BOD per hectare metre per day. The total hydraulic loading rates may range from 110 to 330 Mld per hectare. The BOD removal in single stage biofiltration may be about 80 to 90%, while two stage biofiltration may produce about 95% BOD removal.

2. Accelo-Filters:

These are relatively deep filters with depth 1.8 to 2.4 m. These filters utilize the direct recirculation of unsettled filter effluent to the distributor feed. These filters use 100 mm size stones as filter media and hydraulic loading rates are maintained at less than 340 Mld per hectare of filter area. Fig. 14.10 shows some of the arrangements used in Accelo-filter plants.

As with biofilters, in the case of Accelo-filters also a large variety of flow diagrams is possible, including use of a primary high-rate filter and a secondary low-rate filter. The system of recirculating the effluent from the bottom of the filter directly back to the top of the filter provides a means of reducing odours, increasing the concentration of dissolved oxygen and continuously reseeding the influent to the filter with active organisms.

The organic loading rate in single stage operation may be of the order to about 9000 kg of 5-day BOD per hectare metre per day, with recirculating rates of 100 to 200 per cent of raw sewage flow.

3. Aero-Filters:

In aero-filters sewage is applied continuously and uniformly over the filter bed at a relatively low rate. Some aero-filters are provided with fans that are used to draw air through the filter. Whereas, in the biofilters and accelo-filters there is intimate contact of sewage with the biological film of organic matter formed within the particles of the filter media, in the aero-filters, on the other hand, the same results are obtained by the use of special type of distributors designed to provide application of sewage in the form of rain drops.

Recirculation is used only during periods of low sewage flow, and that too only in amount necessary to ensure proper operation of the distributors. Fig. 14.11 shows the flow diagrams of aero-filter plants. Ordinarily single stage treatment is used. However, if additional treatment is required to lower the BOD of the effluent a second stage filter may be used, and with very strong sewage intermediate clarifier may be provided.

Aero-filter beds are usually more than 1.8 m deep. The recommended organic loading rate may range from 11000 to 12000 kg of 5-day BOD per hectare metre per day. The hydraulic loading rate, for proper functioning of filters, should not be less than 150 Mld per hectare of filter which may need recirculation during low flow rates.

Merits and Demerits of High-Rate Trickling Filters:

The following are the merits of high-rate trickling filters:

(i) Initial cost is less as smaller volume of filter is required due to high rate of loading on the filter.

(ii) Operating costs are low.

(iii) There is absence of bad smell or odour.

(iv) There is absence of trickling filter flies.

(v) The thin gelatinous film produced at the top is found to supply more continuous food to the aerobic bacteria.

(vi) Working is flexible and hence, the variations in the strength and character of sewage do not seriously affect the efficiency of filter.

The following are the demerits of high-rate trickling filters:

(i) The effluent is not highly nitrified. It, therefore, requires more volume of water for dilution.

(ii) Raw sewage cannot be treated and the process requires primary treatment of sewage.