Biofiltration

1. BIOFILTRATION

2. INTRODUCTION Any type of filter with attached biomass on the filter-media can be defined as a Biofilter . The filtration process in which the pollutants are removed due to Biological degradation rather than physical straining as is the case in normal filter, is called Biofiltration .

3. Biological Processes involed in B iofiltration

4. 1. Attachment of Microorganism Organisms attached to medium and grow into dense films of viscous, jellylike nature. Mechanisms by which microorganisms can attach and colonize on the surface of the filter media of a biofilter are: (i) Transportation ,( (a) diffusion ( Brownian motion), (b) convection, (c) sedimentation due to gravity, and (d) active mobility of the microorganisms) ( ii) Initial adhesion , (depending upon the total interaction energy) (iii ) Firm attachment (depend on influent characteristics (such as organic type and concentration) and surface properties of the filter media) (iv) Colonization (depend on influent characteristics (such as organic type and concentration) and surface properties of the filter media)

5. 2. Growth of Microorganism Dissolved organics passes into biofilm due to concentration gradients within film. Suspended particles and colloids retain on sticky surface and decomposed into soluble products. Oxygen for aerobic reactions , provided by dissolved oxygen in WW and from void spaces of medium. Waste products diffuse outwards and carried away by water currents. The factors that influence the rate of substrate utilization within a biofilm are (i) substrate mass transport to the biofilm, ( ii) diffusion of the substrate into the biofilm, and ( iii) utilization kinetics within the biofilm .

6. 3.Decay and detachment of microorganisms As the film grows thicker, both anaerobic and endogenous mechanisms occur at biofilm-medium surface interface. Attachment mechanism weakened. Shearing action of WW washes it away. This Process is called “ sloughing”. Sloughing is function of hydraulic and organic loading rate. Limiting factor is oxygen diffusivity. Aerobic zones are limited to 0.1-0.2 mm thickness.

7. Biofiltration Process

8. Factors affecting Substrate Removal WW flow Rate Organic loading rate Rate of diffusivity of substrate Rate of diffusivity into oxygen Temperature

9. Contact Beds for biofiltration 1.Works in cycles(alternate contact of sewage and air). 2.One cycle lengh =8-12 hrs 3. 80% SS and 60-75% BOD removal. 4.Rate of loading should not exceed 110 litres per day per sqm . 5. Have become almost obsolete these days. Source: Environmental engineering, S.K. Garg

10. Intermittent Sand Filters 1.Sewage is applied for 24hrs on one bed then to another bed. 2.D10 =0.2-0.5 mm & Cu= 2to5 . 3.Excellent performance effluent SS<10ppm,BOD<5ppm. 4.Large area requirement. 5. Not used these days. Source: Environmental engineering, S.K. Garg

11. Trickling filters A TF consists of a permeable media made of a bed of rock , slag, or more recently, plastic over which wastewater is distributed and trickles through.

12. Single stage TF Plant PC SC TF PC SC PC SC TF A B C TF

13. Two- Stage TF Plant PC SC TF PC SC TF TF TF SC PC SC TF TF SC

14. Schematic Diagram of Trickling Filter Source: Water Environment Federation (1996),

15. Typical View of Trickling Filter

16. Types of filters Source: Environmental engineering, Metcalf & Eddy

17. FACTORS CONSIDERED IN DESIGN Dosing characteristics of the distribution system Choosing the right packing / media Configuration of the underdrain system Provision for adequate ventilation

18. DISTRIBUTION SYSTEM Consists of distributor arms mounted on a pivot in the center, These are hollow through which water is discharged over the filter bed For uniform distribution the flowrate per unit of length should be proportional to the radius from the center Head loss is in the range of (0.6 –1.5 m) Dosing tanks must ensure that minimum flow is sufficient to rotate the distributor & discharge all wates from nozzles

19. FILTER MEDIA A material having high surface area per unit volume , does not clog easily Media can be rock , plastic , pvc etc. Stones less than 25mm not be used as they do not provide sufficient pore space

20. DIFFERENT TYPES OF FILTER MEDIA Source: Environmental engineering, Metcalf & Eddy

21. UNDERDRAINS & AIRFLOW These are waste water collection systems that collects the filtered waste water & solids discharged from the filter packing They ventilate the filter by providing air for the micro-organisms by being open at least to a circumferential channel Should be designed such that forced air vibrations can be added later Air flow: Natural draft Forced air ventilator

22. UNDERDRAINS Source: Environmental engineering, S.K. Garg

23. ADVANTAGES Because of their large air-water interface can remove CO 2 ,H 2 S, N 2 & other gases A portion of liquid in underdrain system is recycled: It improves the treatment efficiency To dilute strength of incoming wastewater To maintain enough wetting of slime layer Prevents ponding in filter Suitable for shock loads Low running cost

24. DISADVANTAGES Additional treatment may be needed to meet more stringent discharge standards Generates sludge that must be treated and disposed of Regular operator attention needed Relatively high incidence of clogging Relatively low loadings required depending on the media Limited flexibility and control in comparison with activated sludge processes Potential for vector and odour problems

25. Nitrification in TF Trickling filters running at low organic loading are found to have nitrification( Stequist et al. 1974; Parker and Richards, 1986) Heterotrophic bacteria have high yield coefficient and high growth rate are competitive than nitrifying bacteria Significant nitrification occurs when BOD is appreciably reduced. BOD had to be less than 30 mg/L to initiate nitrification and less than 15 mg/L for complete nitrification(Bruce et al., 1975)

26. OPERATIONAL TROUBLES Fly nuisance Odor nuisance Ponding trouble Icing

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