A

^^Rock Madia

Plastic Madia^ Q

15 20

Figure 5.7 Ammonium removal efficiency for rock and plastic filters at various sites in the USA, applying different amounts of organic loading per unit of surface in kg BOD / 1000 m2 * day. After Parker and Richards (1986).

The EPA (1975) manual showed the removal rate for the NTF to be between 0.83 and 1.50 g N / m2 * d. A conventional design practice has been to follow the NTF with either effluent filtration or clarification.

Recognizing the costs advantages of operation and maintenance of NTF technology, studies have been undertaken to assess the factors limiting the possible nitrification rates, and to modify the processes of the NTF.

As a result of those studies Parker et al. (1989) proposed the Biofilm-Controlled-Nitrifying-Trickling-Filter (BCNTF). The new design incorporated weekly flooding and backwashing of the BCNTF for predator control, and cross-flowed plastic media were applied for better oxygen transfer to the biofilm, resulting in a higher biomass content. The peak nitrification rates obtained for the BCNTF were between 2.3 and 3.2 g N /m2 • d (0.32 and 0.44 kg N/ m3 " d). The BCNTF process has therefore, a peak nitrification rate of about 3 times the NTF process.

Figure 5.8 Relationship between nitrification and soluble BOD5 level exposed to the biofilm for cross-flow media; After (Parker and Richards 1986).

Figure 5.8 Relationship between nitrification and soluble BOD5 level exposed to the biofilm for cross-flow media; After (Parker and Richards 1986).

Additional advantages of the BCNTF is the smaller land area needed and that it can be constructed without disruption of secondary treatment operations. These changes in design and other improvements have made the BCNTF very competitive with the nitrifying activated sludge process.

Denitrification with a trickling filter.

Trickling filters are also able to conduct denitrification, when part of the filter has low oxygen concentration, the presence of nitrate and a carbon source that can act as an electron donor in the denitrification process. Effluent recycling is predicted to be favourable to the denitrification process. Almost all NTF units can denitrify a part of the formed nitrate-nitrogen, depending on the circumstances mentioned above. Stenquist etal. (1974) mentioned an example where a combined trickling filter loaded with 0.36 kg BOD / m3 * d, caused denitrification of 25 % of the ammonium-nitrogen applied to the plant, and 89 % of the ammonium-nitrogen applied was nitrified.

5.6.4 Recent Developments in the Technology of the Nitrifying Trickling Filters (NFT)

The development of the Biofilm-Controlled-Nitrifying-Trickling-Filter (BCNTF) (see Fig. 5.9) is the latest effort to enhance the nitrification rate in nitrifying trickling filter technology. The BCNTF has a peak nitrification rate of about three times that of a conventional NTF. The suspended solids (SS) from the effluent from a BCNTF are almost the same as those found in the influent. If the existing secondary effluent, therefore, is already of a high quality (i.e. the average effluent SS and BOD are less than 15 mg/l) it has been shown in the literature that applying BCNTF is less costly than using a conventional activated sludge process. Further information about the BCNTF is presented above.

5.6.5 Nitrogen Loading Capacity and Removal Efficiency of the Different NTF-applications

Gulliecks and Cleasby (1986) proposed the curves shown in Figs 5.1 OA and 5.1 OB as design curves for application of nitrification to municipal secondary effluent, which has been settled before use of the trickling filter. The filter used for these design curves contained 6.55 m of vertical-type plastic media with a specific surface area of 88.6 m2/m3. Figure 5.1 OA is proposed for waste water with a temperature below 10 °C, and Fig. 5.1 OB for temperatures between 10 and 14 °C.

The curves correlate the influent ammonia-nitrogen concentration, the applied hydraulic flow in l/m2 * s, with the expected yield in nitrification rate in kg N/m2 * d for the trickling filter. It is important to note that the maximum range for the influent ammonia-nitrogen and hydraulic flow on the axes of the curves. If the concentrations in a sample of waste water exceed the values on the axes of curves presented in Figs 5.1 OA and 5.1 OB, it is then necessary to use recirculation in order to achieve a mixed concentration, which is applied to the proposed curves for the use of the curves in estimations.

TRICKLING FILTER / SOLIDS CONTACT PROCESS (TF / SC)

Primary effluent

TRICKLING FILTER / SOLIDS CONTACT PROCESS (TF / SC)

Primary effluent

Lift station

Return sludge

Waste sludge

Figure 5.9 Applying Biofilm-Controlled-Nitrifying-Trickling-Filter (BCNTF) to process application of a conventional trickling filter. After Parker et at. (1989).

Lift station

BIOFILM CONTROLLED NITRIFYING TRICKLING FILTER (BCNTF)

J Nitrifying | trickling filter i

I ^

En,

I

I

Effluent to i disinfection or

I decharge

Effluent to i disinfection or

I decharge

Return sludge

Waste sludge

Figure 5.9 Applying Biofilm-Controlled-Nitrifying-Trickling-Filter (BCNTF) to process application of a conventional trickling filter. After Parker et at. (1989).

Figure 5.11 shows that there is a great variation of the peak nitrification rate at different depths in an NFT.

This decline is attributed to the patchy development of the biofilm at greater depths, caused by the absence of a continuous supply of ammonia to support biofilm development at such depths. Most peak nitrification rates are, therefore, calculated for the whole NTF, and not at certain depths in an NTF.

Table 5.7 shows the peak nitrifying rate for the main types of nitrifying trickling filters. The Table indicates that the BCNTF system developed by Parker and coworkers yields a peak nitrification rate of 2,3 to 3,2 g N / m2 * d, which is high compared with previously developed NTF's.

Stenquist et al. (1974) reported that up to 25 % of denitrification (complete nitrogen loss) were found in NTF plants, depending on the design, as indicated in Section 5.6.1.

Applied NH„* - N mg/l. (including recirculated NH4+ - N)

Applied NH„* - N mg/l. (including recirculated NH4+ - N)

Applied Hydraulic Load I / s " m2 of Cross Section (including recycle)

Applied NH/ " N m9/l' (including recirculated NH4+ - N)

Applied NH/ " N m9/l' (including recirculated NH4+ - N)

Applied Hydraulic Load I / s • m2 of Cross Section (including recycle)

Figure 5.1 OA and 5.1 OB The predicted removal of kg N / m2 * day of the media surface, versus the applied hydraulic load and applied ammonia-nitrogen for nitrification of a municipal secondary clarifier effluent at a waste water temperature below 10 °C (A) and between 10 ° C and 14 °C (B). After Gulliecks and Cleasby (1986).

Nitrification Rate,

Nitrification Rate,

Ammonia Nitrogen Cone., mg/l

Figure 5.11 The nitrification rate as a function of ammonia concentration at four different depths in a trickling filter. After Parker et al. (1989).

Table 5.7 The results from different NFT's as presented in the literature.

Organic loading

Trickling filter media

Performance total possible

Reference

0,16 kg BOD5/m3 d

Rock

75%

EPA (1975)

0,64 kg BOD5 /m3- d

Rock

10%

EPA (1975)

0,36 kg BOD5 /m3- d

Plastic media

89% nitrification 25% denitrification

Stenquist etat. (1974)

2,5 kg BOD5 /1000 m2-

d

Plastic Cross flow media

92%

Parker etat. (1986)

2,5 kg BOD5 /1000 m2-

d

Plastic vertical flow media

60%

Parker (1976)

6,3 kg BOD5 /1000 m2-

d

Plastic media Garland Texas, USA

42%

Parker etat. (1986)

Table 5.8 Comparison of nitrification rates for different NFT plants.

Location

Media

Temp. °C

Nitrification

e'

Reference

Midland,

Vertical flow

13

1,20

0,86

Duddles G.A. et al. (1974)

Mich

media

7

0,93

0,74

H

Lima, Ohio

Vertical flow

21

1,70

1,01

Sumpayo E.F.(1973)

Bloom Township

Vertical flow

20

1,20

0,88

Baxter and Woodman (1973)

III

media

17

1,10

0,82

Zurich Switz.

Cross flow

17-20

1,40

0,65

Richards (1988)

(3.9 m tower)

plastic media

Zurich Switz.

Cross flow

13

1,10

0,39

■I

(6,8 m tower)

plastic media

Central Valley

Cross flow

18

2,60

0,80

Parker et al. (1989)

plastic media

= Media Effectiveness factor (E).

= Media Effectiveness factor (E).

5.6.6 Advantages and Disadvantage of the NTF

The following advantages and disadvantages can be listed for the application of a nitrifying trickling filter.

Advantages:

* Their simplicity and low operational cost make the trickling filters an attractive option for small communities in warmer climates.

* The recovery from hydraulic and substrate shock-loads is fast.

* It is possible to obtain a high content of biomass, especially when highly porous plastic media are used.

Disadvantages:

* Trickling filters achieve only with difficulty the high efficiency which is demanded by recent effluent standards in many countries.

* Most trickling filter effluent needs a polishing process, because the concentration of suspended matter at high loadings is unacceptable for meeting effluent standards.

* It is difficult to ensure an effective predator control, so the maximum nitrification rate can rarely be obtained.

5.7 Rotating Biological Contactors (RBC)

The Rotating Biological Contactor (RBC) is used for a variety of purposes: Aerobic degradation of organic material; combined organic removal and nitrification; and denitrification and nitrification of secondary and tertiary effluent (after filtration).

A rotating biological contactor (RBC) consists of a series of closely spaced rotating circular discs made of different kinds of materials, for example plastic, wood, and galvanized plates.

These discs are approximately 40 % immersed in a tank through which waste water flows continuously. The discs are mounted on a shaft which usually rotates through the water at a velocity of 1 rpm. A layer of biological growth, depending on the composition of the waste water, builds up on the wet surface of the discs and forms a biofilm ranging from 1-2 mm in thickness. The formation of a fully developed biofilm takes from 1 to 4 weeks. As the discs rotate through the waste water, the ammonium content is nitrified, and the organic carbon content is oxidized by various microorganisms. Excess growth on the discs is disposed of at the same time. The discarded biofilm Is washed out of the unit and removed during a secondary clarification. As the biofilm is passed out of the liquid and through the air, oxygen is absorbed to keep the growth aerobic.

An RBC treatment plant will generally consist of a number of shaft trains, each operating as a completely mixed, fixed-film biological reactor. Each train is generally set up in a number of stages, separated by baffles for more efficient treatment and stability. By doing so, it is possible to achieve a high degree of nitrification. Figure 5.12 shows a flow diagram of the rotating biological contactor process.

5.7.1 The Performance of the RBC

The factors affecting nitrification in the RBC process are the same as in other nitrifying plants, namely, organic concentration, Influent nitrogen concentration and composition, waste water temperature, DO concentration, pH and alkalinity, and influent flow and load variability.

Most empirical design procedures are based on the assumption that significant nitrification does not begin in an RBC system until the bulk liquid soluble BOD5 has been reduced to 15 mg/l. In combined carbon oxidation-nitrification units this will typically first be encountered in the third or fourth stage, depending on strength, organic loading rate and temperature of the influent.

Hydrogen ions are produced during nitrification. In poorly buffered RBC nitrifying systems, alkaline chemicals such as lime, soda ash, and sodium hydroxide may have to be added to the waste water in order to maintain a sufficient alkalinity to prevent a sudden decrease in pH and thereby a decrease of the nitrification rate.

Rotating Biological Contactor (RBC units)

Primary clarifier Final Clarifier

Rotating Biological Contactor (RBC units)

Primary clarifier Final Clarifier

treatment

Figure 5.12 Flow diagram for the rotating biological contactor process.

treatment

Figure 5.12 Flow diagram for the rotating biological contactor process.

The media in an RBC.

The RBC media must serve several purposes according to Antonie (1976):

It must:

1. provide a surface area for the development of a large, fixed, suitable biomass,

2. provide vigorous contact of the biological growth with the waste water,

3. aerate the waste water efficiently,

4. provide a positive means of continuously removing excess biomass, and

5. agitate the mixed liquid to keep the discharged solids in suspension and thoroughly mix each stage of treatment.

Many different materials for RBC media have been used over the years, from the wooden slats of Poujelet in 1916 to the plastic discs used today. The discks are usually 2-3 m in diameter and 1.2 cm thick.

Today the discs are made from a high-density polyethylene in alternating flat and corrugated sheets, which are bonded together. This design provides more than twice the surface area per unit volume than flat sheets.

The rotational speed of an RBC.

The rotation of discs serves a varity of purposes in the RBC process:

1. It provides a contact between the biomass and waste water.

2. Removal of excess biomass.

3. Mixing of the liquid and aeration of the waste water.

If the rotation rate is increased, the effects mentioned above are enhanced to a point above which further increase is not productive.

The optimum rotational speed for the RBC varies depending upon the composition of the waste water and the disc size of the RBC.

In practice, most RBC units are operated at 1.0 to 1.4 rpm.

The aeration of an RBC.

In some RBC facilities, aeration equipment has been installed, either to drive the RBC shafts or to provide supplemental aeration. RBC with aeration facilities usually results in a thinner biofilm on the discs in the first compartments because of the stripping action of the bubbles, thereby allowing more of the biofilm to remain aerobic.

It appears, however, that the dissolved oxygen in the mixed liquid has little effect on the transfer of oxygen into the biofilm (see Fig. 5.13). A study of the mass transfer of oxygen in the biofilm indicates that very little of the oxygen utilized by the microorganisms in the film comes from the bulk liquid in the RBC tank; it comes from the atmosphere, when the disc surface is exposed to air. The transfer of oxygen to the biofilm is better increased by lengthening the exposure time to air or reducing the thickness of the liquid film on the disc by more efficient emptying than by aerating the waste water. Figure 5.13 show the relative concentrations of oxygen and substarte for the loading condition and RBC rotational spped as a function of the media.

.Direction of Aerated^X process sector

Submerged 'J C sector _

SRBC plastic media

Distance from RBC

IN ATMOSPHERE

IN BULK LIQUID

RBC plastk medi*

0 0

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