Waste water treatment: Trickling filter

Trickling filter at Al Aweer WWTP Biological Treatment Trickling Filter Trickling Filters Anaerobic • Whereas activated sludge is a suspended growth process, a trickling filter is an attached growth process Aerobic Air Filter medium • Wastewater trickles over medium, bacteria grow on medium, creating biofilm • Biofilms are the means of treatment in trickling filters • The packing of a trickling filter could be rock (old) or plastic (recently used) material • Substrate, O2 and nutrients diffuse across a stagnant boundary layer Making the biodegradation reaction diffusion limited Wastewater flow Biological layer Liquid film Trickling Filters versus Activated Sludge Advantages • Less energy needed • Simpler operation • Less operation and maintenance Disadvantages • Poorer effluent quality • Sensitive to low temperature • Produces odors • Nitrogen removal is difficult Types of Trickling Filters Low rate or standard rate • Rock filters: 1-3 m deep • Loading: 2-20 lb BOD/1000 ft2.day High rate • Rock or plastic filters: 1-2 m deep • Loading: 20-60 lb BOD/1000 ft2.day Super rate • Synthetic packing • Loading: 50- 380 lb BOD/1000 ft2.day Raw water Primary settling )Q( )Recirculated flow (QR Secondary settling One-stage Trickling Filter Raw water Primary settling Treated )water (Q Trickling filter Treated )water (Q )Recirculated flow (QR Intermediate settling tank )Recirculated flow (QR Final settling tank Trickling filter Two-stage Trickling Filter Trickling filter Treated water Design Parameters Parameters needed for the design of a Q trickling filter system include BOD loading, hydraulic loading and recirculation ratio These parameters are : defined as follows BOD loading = )Recirculated flow (QR Trickling filter settled BOD (lb / day ) volume of filter Hydraulic loading = HL = QR R= Q Primary settling Q + QR A Typical loading for trickling filter with a 5-7 ft depth of stone or slag media BOD loading (lb/1000 ft3/day) 30-90 HL (gpm/ft2) 0.16-0.48 Recirculation ratio (R) 0.5-3.0 BOD Removal Efficiency ET = E20 (1.035)(T − 20) Example A one-stage trickling filter has a primary clarifier (55-ft dia, 7-ft depth, single peripheral weir), trickling filter (85-ft dia, ft stone filled bed), and final settling tank (50 ft dia, ft side water depth single peripheral weir) R=0.5, Q= 1.38 mgd, and average BOD = 180 mg/l • For the primary clarifier determine overflow rate, detention time and weir loading • For the trickling filter determine the BOD and hydraulic loading • What is the effluent BOD at 16 oC Primary settling )Recirculated flow (QR Secondary settling Trickling filter Treated )water (Q Solution For the primary clarifier: Overflow rate= vo=Q/As = 1,380,000/2370= 589 gpd/ft2 Detention time= t= V/Q= (0.124 mil gal/ 1.38 mgd) *24hr/d = 2.2 hr Weir loading= Q/Lweir= (1,380,000 g/d)/(3.14*55)=8000 gpd/ft For the trickling filter: A= 3.14* (85)2/4= 5660 ft2 V= 5660*7= 39,600 ft3 HL= (1.38 +0.5*1.38)*106/ 5660 = 366 gallon/ft2/day BOD load: Assume 35% BOD removed by the primary clarifier, then settled BOD is 0.65*180= 117 mg/l BOD loading = = settled BOD (lb / day ) volume of filter 1.38mgd *117 mg / l * 8.34 = 34 lb BOD / 1000 cu ft / day 39.6thousand cu ft Using Fig 11-19 with BOD load =34 and R =0.5, we find the BOD removal efficiency is 78% at 20oC Now using ET = E20 (1.035)(T − 20) We find the efficiency at 16 oC = 68% Thus, the effluent BOD will be 117 (100-68)/100= 37 mg/l