PROCESS Depending on the design, BOD removals from 50 to 90% can be expected. Dikes protected from erosion by use of riprap (we have even used broken concrete from sidewalk repairs in our city).
PONDS
Reliable treatment; usually lower capital and operational
costs than "tank treatment process"; usually used when
land is available and less expensive than "tank treatment";
mostly flows less than 1 MGD; rural, small towns with less expensive
land.
PROCESS
Depending on the design, BOD removals from 50 to 90% can be expected.
Dikes protected from erosion by use of riprap (we have even used broken
concrete from sidewalk repairs in our city).
Ponds are usually categorized according to the presence/quantity of oxygen in the pond water: a) aerobic, b) facultative, c) anaerobic, and d) tertiary. Also categorized by use: a) Controlled discharge, b) Un-controlled discharge.
AEROBIC PONDS
Are large, shallow lagoons with a water depth of 0.5 feet to 1.5 feet
to maximize the growth of algae or at a depth of 3.0 feet to 5.0
feet to maximize the production of oxygen by algae. Addition of
oxygen by wind action on the surface is very important. Best results
for treatment is when the contents are mechanically mixed on a
periodic basis. Bacteria consume organic material and give off
chemical compounds. Algae utilize many of the chemical compounds
in their metabolic processes, and give off oxygen during photosynthesis
for the aerobic bacteria.
FACULTATIVE PONDS
These ponds have
50 to 60 Days DETENTION TIME; Reduce BOD by 70 to 80%, and coliform reductions
from
90 to 95% about 80% of the time. Posses three layers of operation:
Top Layer is aerobic, Middle Layer is facultative, and Bottom
Layer is anaerobic. Top layer: aerobic bacteria consume organic
matter release ammonia (NH3), carbon dioxide(CO2), water, etc.
Algae use the carbon dioxide, ammonia, phosphates (PO4) in photosynthesis
and metabolic processes and release oxygen during photosynthesis.
We use aerators, under periods of high loadings, to increase the
oxygen in the aerobic zone. Middle Layer fluctuates between aerobic
and anaerobic conditions. Facultative bacteria oxidize organics
in this minimal/no oxygen layer. Bottom layer: settled solids
are consumed by anaerobic bacteria. Bacteria release hydrogen
sulfide (H2S), carbon dioxide (CO2), and methane (CH4) "swamp
gas" into the water. These gases are released into the atmosphere
from the water or are utilized by the algae (depending on many
factors).
ANAEROBIC PONDS
Usually quite high strength, high grease/oil loading rates i.e./
meat processing packers. Used to treat the "solids",
as opposed to the "water & some solids" in aerobic
and facultative ponds. Grease/scum layer maintained in most for
better insulation. Lagoons are deep, up to 30 feet, with steep
sides to help maintain anaerobic conditions, and retain heat.
Effluent is usually treated in a separate process, like an aerobic
pond. BOD reduced from 70% up to 85%. Many have plastic type covers on them for
additional insulation.
TERTIARY PONDS
Also called polishing ponds, and are very lightly loaded with organics. Detention
times of 30 days or more are used to provide time to consume/reduce solids.
May be used to nitrify plant effluent (ammonia converted into
nitrite by nitrosomonas bacteria, and then the nitrite is converted
into nitrate by nitrobacter bacteria. See the Nitrification Chapters for more
information).
CONTROLLED DISCHARGE PONDS
These have about
180 Day DETENTION TIMES which will remove 85-95% of the BOD, and 99%
fecal coliform removal rate.
OPERATION NOTES
If increase in loading, must increase oxygen transfer; must maintain
oxygen level in ponds. Add mechanical aeration, especially at
night when photosynthesis not occurring. Depth of each layer depends
on the temperature, the season of the year, organic loading type/length
of time of mechanical aeration. If algae settles out in pond,
BOD stays in the pond as algae raise BOD value when they die off
during BOD lab test. Higher than desired amounts of suspended
solids (mostly algae & bacteria) and BOD in the effluent may
be reduced by a careful dosage of a polymer or alum to the influent
to the last pond, if a series of ponds exits. This will help settle
the algae and bacteria, as in a clarifier. I have also used baffles
to retain solids in the pond, by forcing the pond effluent to
come from under the baffle, four feet below the surface, where
there is less algae, and suspended solids. Evaporation & percolation
may be high enough to reduce effluent from the ponds. City of
Dixon, California has wastewater treatment/containment pond system
with zero wastewater effluent discharge. Evaporation may account
up to 0.33 inch per day or more water loss alone. Most ponds are
2 to 5 feet deep with continuous flow through the pond. Organic
loadings of 400 persons per acre; less than 12 lb BOD/day/surface
acre for tertiary; 15 to 60 lb BOD/day/acre for aerobic high rate
ponds with detention times to 30 days. Facultative may range from
15 BOD/day/acre & 10 days detention time to 65 lb BOD/day/acre
and 30 days detention time. Depends on temperature, sunlight,
type of BOD, etc. Many, like City of Vacaville's Gibson Plant,
and Easterly Plant have monitoring wells around the perimeter
of the plant for ground water testing. If the groundwater level
is high, or high amounts of percolation occur, it may be best
to line the pond with a plastic or rubber liner to reduce the
transfer of nitrates in the pond water into the groundwater, etc.
Control weeds, cattails, and mosquitoes. Slowly raising and lowering
the pond levels discourages muskrats burrows (they like partially
submerged entrances). Ground squirrels weaken the cell walls by
burrows.
MATHEMATICS
Aerobic and facultative ponds are sized and operated on the basis
of: Pounds BOD/day/acre. Anaerobic ponds are sized and operated
more like digesters: Pounds BOD/day/ac-ft or pounds BOD/day/cu-ft.
Aerobic and facultative ponds REQUIRE the surface area for oxygen
transfer and algae to produce the required oxygen for the microbes,
so the surface area is the most important parameter. Anaerobic
ponds require the detention time to convert the SOLIDS, so VOLUME
is the important parameter.
INTERACTIONS
IF THE POND IS CREATING VISUAL OR ODOR NUISANCE, HAS A HIGH BOD,
SOLIDS, GREASE, OR COLIFORM DISCHARGE, IT IS NOT REGARDED AS PERFORMING
PROPERLY. As the sunlight intensity increases, the algae use more
carbon dioxide than they give off, (which they use in photosynthesis),
which makes the pH increase (less carbonic acid, see below). In
sunlight the algae also make more oxygen than they use, which
makes the dissolved oxygen content go up in the water. As the
sunlight decreases, algae decrease photosynthesis which means
giving off less oxygen, and less carbon dioxide is used. When
darkness sets in, no oxygen is given off, and carbon dioxide is
given off in respiration. The pH therefore decreases, and the
dissolved oxygen (DO) decreases as the algae, bacteria, and the
other organisms use the oxygen. <<CHEMISTRY: Carbon dioxide
combines with hydrogen atoms already in the water, to make carbonic
and carboxylic acids. The addition of these acids, reduce the
pH of the water (makes it more acidic.>> Oxygen solubility
is higher in colder water than in warmer waters. The anaerobic
reactions in the bottom of the facultative ponds, and the anaerobic
ponds is very much like an anaerobic digester: acid forming bacteria
break down the organics converting them into organic acids like
acetic and formic acid; methane producers convert the simple organics
and acids into methane gas, carbon dioxide, and hydrogen sulfide.
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