This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
a. Algal uptake at the upper surface - The surface of the bed may support algal growth when temperature and sunlight
conditions are optimum. - Do not take a direct part in waste degradation, but add oxygen
- troublesome because they can cause clogging of the filter surface (which produce
odors).
b. Nitrification at the bottom
- The lower portion of a deep filter frequently supports populations of nitrifying
bacteria.
c) Higher organisms
- Snails are troublesome, they consume nitrifying bacteria.
4. Growth Phases
a. Exponential growth phase near surface of the bed.
- Microorganisms near the surface of the bed, where food concentration is high, are in
a rapid growth phase.
b. Declining death phase near the bottom. - The lower zone of a bed is in a state of starvation. c. Endogenous growth - Overall operation of a trickling filter may be considered in the endogenous growth
phase.
7-Trickling Filter_F11.doc
5
b. Filter media 1) provide:
i) a surface for biological growth and ii) voids for passage of liquid and air. 2) the most common media are: i) crushed rock, ii) slag, or iii) field stone - these materials are durable, insoluble, and resistant to spalling. 3) the preferred range of size for the stone media is: i) 3 - 5 in. in diameter (VH) ii) 1 - 4 in (25 - 100 mm) ME, p 404
depth of rock bed = 3 - 8 ft (0.9 - 2.5 m) = 6 ft (1.8 m avg) ME p 404
Natural
draft air
100 ft (30 m)
Wastewater id distributed
by hydraulic head
Filter media 5-7 ft
Underdrain
Effluent
Influent Underdrain collects treated WW
and supply air (O2)
7-Trickling Filter_F11.doc
6
4) plastic media (Fig. 12.32, VH., p.479; Fig. 12.33, p. 481) i) depth = 14 - 40 ft (4 - 12 m) ME p. 404 ii) types vertical-flow packing (Fig 8-23); cross-flow packing (Fig 10-33); variety of random packings (Fig 10-33) iii) have advantages of: . light weight . chemical resistance . high specific surface (ft
2/ft
3 or m/m
3) with a large percentage of free space.
c. Underdrain system 1) Carries away the effluent - collecting the treated wastewater and any biological solids that have become
detached from the media - carries away the effluent 2) Circulates air - permits circulation of air through the bed. - the need for free passage of air controls the size of opening in the underdrain. 3) Recycle line - to dilute the strength of the incoming wastewater - to maintain the biological slime layer in a moist condition.
7-Trickling Filter_F11.doc
7
d. Rotary distributor 1) Provides uniform hydraulic load - provides a uniform hydraulic load on the filter surface. 2) Require pressure head (minimum 24 inches) - it is driven by the reaction of the wastwater flowing out of the distributor
nozzles. - requires a minimum pressure head of 24 in. measured from the center of the
arms. 3. Operation a. Spray - primary effluent is sprayed on a bed of media (e.g., crushed rock or plastic
media) coated with biological films. b. Microbial metabolism - As the wastewater flows over the microbial film, the soluble organics are rapidly
metabolized and the colloidal organics adsorbed onto the surface. c. Reoxygenation - Dissolved oxygen extracted from the liquid layer is replenished by reoxygenation
from the surrounding air. d. Sloughing - losing the slime layer - a function of the organic and hydraulic loading on the filter. 4. Operational problems a. Undesirable anaerobic conditions - Undesirable anaerobic conditions can be created in a trickling filter by inhibiting
aeration of the bed. b. Plugging - Plugging of the air passages with excess microbial growth, as a result of organic
overload, can create anaerobic and foul odors. - organic overload 5. Air movement 1) (summer) If air temperature > waste temperature, Tair > Tw, air moves down through the filter bed 2) (winter) If air temperature < waste temperature,
Tair < Tw, air moves up through the filter bed 3) (spring & fall) If air temperature = waste temperature, Tw = Tair, no ventilation, no air movement * June and august are transition zone, then system become anoxic
7-Trickling Filter_F11.doc
8
6. Advantages and Disadvantages of TF Advantages: a. Low energy input b. accepts qualitative and quantitative shock load very well c. accepts toxic load well d. good sludge settling at secondary clarifier Disadvantages: a. land requirement - much land needed b. performance 1) fair in summer – mediocre, 30 mg/L BOD in effluent 2) poor in winter - can increase performance by covering . cost for cover . corrosion . condensation 5. Level of Treatment a. Secondary treatment systems 1) Normally, TFs are proceeded by primary treatment 2) the standard stone-filled filter must be preceded by a sedimentation tank equipped
with a scum-collecting device. 3) the system includes a final settling tank to remove biological growths that are
washed off the filter media. 4) the sloughed solids are commonly disposed of through a drain line from the bottom
of the final clarfier to the head end of the plant. 5) this return sludge flow is mixed with the raw wastewater and settled in the primary
clarifier. 6. Classification of TF (ME, p. 615, Table 10-13)
- TFs are classified by: a) hydraulic loading rate
b) organic loading rate Classifications are: 1) Low-rate or standard rate (Fig. 12.26, VH p.466) 2) Intermediate-rate 3) High-rate (Fig 12.27, VH p.467) 4) Super high-rate 5) Roughing filters 6) Two-stage (Fig 12.29, VH p. 468)
or 1) Single-stage 2) Two-stage
7-Trickling Filter_F11.doc
9
Low-rate TFs (Fig. 12.26, VH p.466) - or Standard Rate 1) rarely used in new designs 2) the wastewater passes through only once 3) the effluent is then settled prior to disposal. 4) the sludge line is operated, generally once or twice a day, to waste the accumulated
settled solids. 5) operate intermittently, dosing and resting. This operation is required bacause of the
low hydraulic load on the filter. - low night time flow may result in intermittent dosing
6) a dosing siphone, or similar alternating flow-control system, must be installed ahead of the filter to provide an adequate flow rate to turn the reaction-type rotary distributor.
- otherwise, the distributor arm would stop turning during periods of low, such as at night, and wastewater would trickle down only under the stalled arm.
- constant hydraulic loading is maintained by suction level controlled pumps or a dosing siphon (not by recirlulation).
Intermediate and High-rate TFs (Fig 12.27, VH p.467)
1) raw wastewater is diluted with recirculation flow so that it is
passed through the filter more than once.
2) a return line from the final clarifier serves a dual function,
as a sludge return and a recirculation line.
3) the combined flow (Q+QR) through the TF is always sufficiently
great to turn the distributor, and dosing siphone is not needed.
4) a high-rate filter is dosed continuously.
Super High-rate TFs
1) high hydraulic loading and great filter depths (ME, Fig 10-32)
- because of the heavy organic loading on roughing beds, their
design must consider the problem of bed plugging.
2) Hydraulic loading is up to 3.2 gal/ft2.min (187 m3/m2.d)
3) Most use plastic media
- the high percentage of void volume in manufactured media permits their application
and use in roughing filters.
4) Treat wastewater prior to secondary treatment.
- those receiving unsettled wastes, as well as other used for
pretreatment of settled strong wastes prior to subsequent
aeration, are referred to as roughing filters.
Two-stage TFs (Fig 12.29, VH p. 468)
a. Two filters in series, usually with an intermediate settling tank.
b. Super high rate-Roughing filters
c. Two-stage filters are used
1) where a high-quality effluent is required,
7-Trickling Filter_F11.doc
10
2) for treatment of strong wastewater
3) to compensate for lower bacterial activity in treating cold
wastwater.
4) where nitrification is required.
Recirculation (ME, p. 617, Figure 10-31)
Figure 10-31 (ME, p. 617) shows intermediate-rate and high-rate trickling-filter flow
diagrams with various recirculation patterns:
(a) single-stage filters and (b) two-stage filters.
1) the recirculation ratio is the ratio of recirculated flow to the
quantity of raw wastewater
2) a common range for recirculation ratio values is 0.5-3.0
QR
R = ------ = 0.5 - 3.0
Q
3) Purposes of recirculation
a. dilute very strong wastes
b. dilute toxic waste
c. minimize Q and BOD flactuations to filters
d. Return sludge for collections
e. seeding effect - seed input w/ some of viable microbes
4) Recirculation Control
- recirculation is done
a. only during periods of low wastewater flow
b. at a rate proportional to raw-wastewater flow
c. at a constant rate at all times
d. at two or more constant rates predetermined automatically
or by manual control.
5) Recirculation Patterns (Fig 12.28, VH p.467)
a. recirculation with sludge return
b. direct recirculation around dilter
c. recirculation of plant effluent
d. dual recirculation.
7-Trickling Filter_F11.doc
11
Loading Standards for Stone-Media Trickling Filters a. Standards for Trickling Filters with Stone, Crushed Rock, or Slag Media (Table 12.2, VH p.469) b. The BOD load on a TF is calculated using the raw BOD in the primary effluent applied to the filter, without regarding to the BOD in the recirculated flow. * Criteria for two-stage filters based on the total volume in both filters. c. BOD loadings are expressed in terms of lbs of BOD applied /day lbs of BOD applied/day ------------------------ or ---------------------- 1000 ft
3 of Volume acre-ft
* The hydraulic load is computed using the raw-wastewater flow plus recirculation flow. d. Hydraulic loadings are expressed in terms of million gallons applied average flow (in gpm) applied ---------------------------- or ----------------------------- acre of surface area per day ft
2 of surface area per day
d. Current values used in design (VH, p. 469, Table 12.2) -------------------------------------------------------------------- Trickling BOD loading Hydraulic loading Recirculation Filter lb/1000 ft
3 g/m
3 gpm/ft2 Ratio
--------------------------------------------------------------------- Low-rate 15 (5-25) 240 0.05 (0.03-0.06) 0 High-rate 35 (25-45) 560 0.25 (0.16-0.48) 1.5 (0.5-3.0) Two-stage 55 (45-65) 880 0.25 (0.16-0.48) 1.5 (0.5-3.0) --------------------------------------------------------------------- In a low-rate trickling filter, the relationship between BOD load and hydraulic load depends on the strength of the applied wastewater. - this relationship exists because there is no recirculation flow and depth of the bed is limited to 5-7 ft. 2. Efficiency equations for stone-media TFs a. National Research Council (NRC) formula
1) Based on data collected from filter plants at military installations in the United
States in the early 1940s.
2) The NRC formula for a single-stage TF
100 E
E = ------------------------ ==> ---- = E1
w 100
1 + 0.0561 (---)0.5
VF
where E = BOD removal at 20oC, %
w = BOD load applied, lb/day
V = volume of filter media, 103 ft
3 (ft
3 x 10
-3)
F = recirculation factor
w
--- = volumetric BOD loading, lb/1000 ft3/day
V
7-Trickling Filter_F11.doc
12
1 + R
F = -------------
(1 + 0.1 R)2
recirculation flow QR
where R = recirculation ratio = --------------------- = ---
raw wastewater flow Q
c. The NRC formula for the second stage of a two-stage TF
100
E2 = ------------------------
0.0561 w2
1 + (------)(----)0.5
1-E1 VF
where E2 = BOD removal of the second stage at 20
oC, %
E1 = fraction of BOD removal in the first stage,
w2 = BOD load applied to the second stage, lb/day
w2
--- = BOD loading, lb/1000 ft3/day
V
c. Effect of wasewater temperature on stone-filled TF efficiency
E = E20 (1.035)
T-20
where E = BOD removal efficiency at temperature T, oC