Trickling Filter | Environmental Engineering - Civil Engineering (CE) PDF Download

TRICKLING FILTER 

• Trickling filter is an attached-growth type of process in which microorganisms attached to a medium are used for removing organic matter from wastewater. that utilizes 
• This type of system is common to a number of technologies such as rotating biological contactors (RBCs) and packed bed reactors (biotowers). These reactors are also called as non-submerged fixed film biological reactors. 

COMPONENTS OF TRICKLING FILTER 

Figure 4.4.1 Trickling filter 

[A] Packing

• Trickling filter uses packing medium composed of crushed stone, slag, rock or plastic over which wastewater is distributed continuously (Figure 4.4.1). 
• The ideal medium should have the following properties: high specific surface area, high void space, light weight, biological inertness, chemical resistance, mechanical durability, and low cost. 
   The important characteristics of medium includes-
   a) Porosity: It is a measure of the void space available for passage of the wastewater and air and for ventilation of product gases.
   b) Specific surface area: It refers to the amount of surface area of the media that is available for biofilms growth.
   c) Size of the medium ranges from 50-100 mm having specific surface area in the range of 50-65 m²/m³ with porosities of 40-50 %.

[B] Wastewater dosing 

• Influent wastewater is normally applied from the top of the trickling filter. · Under a hydraulic head of about 1.0 m, jet action through the nozzles is sufficient to power the rotor.
• As the flow is intermittent, there is enough air circulation through the pores between dosing. 
• The distributer arm distributes the wastewater continuously over the medium, which trickles down through the bed.

[C] Under-drain 

• It is used in trickling filters to support the filter medium, collect the treated effluent and the sloughed biological solids, and circulate the air through the filter.  
• The liquid flow in under-drains and collection channels should not be more than half full for adequate air flows. 

PROCESS DESCRIPTION OF TRICKLING FILTER 

• A rotary or stationary distribution mechanism distributes wastewater from the top of the filter percolating it through the interstices of the film-covered medium . 
• As the wastewater moves through the filter, the organic matter is adsorbed onto the film and degraded by a mixed population of aerobic microorganisms. 
• The oxygen required for organic degradation is supplied by air circulating through the filter induced by natural draft or ventilation.  
• As the biological film continues to grow, the microorganisms near the surface lose their ability to cling to the medium, and a portion of the slime layer falls off the filter. This process is known as sloughing .  
• The sloughed solids are picked up by the under-drain system and transported to a clarifier for removal from the wastewater.
• Microorganisms used
  • The microorganisms used are mainly facultative bacteria that decompose the organic material in the wastewater along with aerobic and anaerobic bacteria. 
  • It includes Achromobacter, Flavobacterium, Psudomonas, and alcaligenes. 
  • In the lower reaches of the filter, the nitrifying bacteria are usually present. 

FACTORS AFFECTING THE OPERATION OF TRICKLING FILTER  

[A] Organic loading

• A high organic loading rate results in a rapid growth of biomass.
• Excessive growth may result in plugging of pores and subsequent flooding of portions of the medium. 

[B] Hydraulic flow rates 

• Increasing the hydraulic loading rate increases sloughing and helps to keep the bed open. Range of hydraulic and organic loading rates for trickling filters are shown in table 1. 

[C] Relative temperature of wastewater and ambient air

• Cool water absorbs heat from air, and the cooled air falls towards toward the bottom of the filter in a concurrent fashion with the water. · Warm water heats the air, causing it to rise through the underdrain and up through the medium. 
• At temperature differentials of less than about 3 to 4⁰C, relatively little air movement results, and stagnant conditions prevent good ventilation. 
• Extreme cold may result in icing and destruction of the biofilms. 

 DESIGN EQUATIONS FOR TRICKLING FILTER 

[A] Tentative method of ten states of USA 

The equation is given as follows: 

 (4.4.1) 
 where, Q is the flow rate, R is the recycle flow rate and E is the efficiency. 

a) Loading rate 

(Raws ettleddomesticsludge)<102kg BOD/(dm³ )

b) R/Q should be such that 

BOD entering filter (including recirculat ion) ≤ 3 x BOD expected in effluents

[B] Velz equation 

The following equation is used for a single-stage system and in the first stage of a two-stage system: 

 (4.4.2)

The following equation is used for the second stage of a two-stage system: 

     (4.4.3) 

 Where, S_e is the effluent BOD from the filter (mg/l), Si is the influent BOD (mg/l), r is the ratio of recirculated flow to wastewater flow, D is the filter depth (m), A is the filter plan area (m²), Q is the wastewater flow (m³/min), T is the wastewater temperature (^oC), k and n are empirical coefficients (for municipal wastewaters, k = 0.02 and n = 0.5) and subscript i (i = 1, 2) repressent the stage number. 

[C] NRC equations

The following equation is used for a single-stage system and the first stage of a two-stage system: 

              (4.3.4) 

 (4.3.5) 

The following equation is used for the second stage of a two-stage system: 

 (4.3.6) 

  (4.3.7) 

[D] Eckenfelder equation (Plastic media) 

The Eckenfelder equation used for plastic media is as follows: ..

 (4.3.8) 

From Eckenfelder equation,