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ACTIVATED SLUDGE
What is it and where did it start?
Activated sludge is a mixture of bacteria, fungi, protozoa and rotifers maintained
in suspension by aeration and mixing. The activated sludge process was
discovered (Fowler, Adren & Lockett) in 1913 in Britain. Their experimentation on
treating sewage in a draw-and-fill reactor produced highly treated effluent. They
aerated the wastewater continuously for about a month and were able to achieve
a complete nitrification of the sample material. Believing that the sludge had been
activated (in a similar manner to activated carbon) the process was
named activated sludge. Not until much later was it realized that what had
actually occurred was a means to concentrate biological organisms, decoupling
the liquid retention time (ideally, low, for a compact treatment system) from the
solids retention time (ideally, fairly high, for an effluent low in BOD5 and
ammonia.) By the late 1930s, the activated sludge treatment was the
predominant process used around the world.
Activated sludge is a biological process that utilizes microbes (bacteria, protists,
microfungi, and microanimals) to convert organic and certain inorganic matter
from a waste stream into cell mass (biomass). The activated sludge is then
separated from the liquid by clarification. The settled sludge is either returned
(RAS) or wasted (WAS).
What is biomass?
Biomass is microbe cell mass (settled activated sludge) – either WAS or RAS.
What is “return activated sludge” (RAS)?
The settled activated sludge (biomass) that is collected in a secondary clarifier
and returned to the secondary treatment process to mix with incoming
wastewater. This returns a concentrated population of microbes back into the
aeration basin.
In a water treatment system what is the food?
BOD – biological oxygen demand. BOD is organic matter in a waste stream.
What do microbes do with this BOD food?
Microbes (bugs) convert dissolved and particulate organic matter (measured as
BOD) into cell mass (biomass). They use oxygen to break down organic matter
(food) for their growth and survival. The more soluble the organic material is, the
more easily microbes can use it. Since the amount and type of organic loading
(amount of food) in the treatment process affects microbe growth.
Over time and as wastewater moves through the aeration basin, food (BOD)
decreases with a resultant increase in cell mass (MLSS concentration).
What environmental factors are important for microbe BOD feeding?
type of food (water-soluble, oil, grease, inorganic, organic etc.) – microbes can
eat anything!
retention time – more time allows more bugs to feed - some bugs need free
oxygen (O2) - typically 1.0-3.0 mg/L - to convert food into energy for growth. no
- other bugs can’t be near free oxygen (O2) to convert food into energy for
growth.
type of food (water-soluble, oil, grease, inorganic, organic)
temperature moderate temperatures (10 – 25oC) are best, but not at all
exclusive!
trace nutrients bugs need trace nutrients such as nitrogen, phosphorus, and
some metals to eat efficiently. The ratio of BOD5 to nitrogen (N) to phosphorus
(P) should be at least 100:5:1.
pH Biological and chemical reactions are affected by pH. Most microbes do
well in a pH environment between 6.0-9.0. Acidic (low pH) or alkaline (high pH)
conditions can adversely affect certain microorganism growth and survival.
toxics certain molecules can be harmful to all but certain microbes.
How are solids created in a traditional wastewater treatment system?
Solids are generated by microbe growth and reproduction. The influent BOD
supplies the food for the growth and reproduction. As microbe populations
multiply, excess solids (bugs) settle.
Can this excess “settling sludge” be reduced?
Yes, incorporation of a BioReactor(s) into waste treatment streams will allow for
significant (40+%) sludge reduction with modification of the waste stream:
presence of fixed biofilm in log-rate growth
sloughing and circulation of active biofilm – increases effective retention times
maintenance of particulate (floc) suspension
recirculation and inoculation within treatment basin(s)
reduction of RAS (return activated sludge)
Do microbes eat microbes?
Yes! The food chain goes like this:
Bacteria are consumed by protists, which are consumed by micro-animals:
PROTISTS:
Ciliates
Flagellates
Amoebas
Micro-Animals
Rotifers
Nematodes
Where is the most energy spent in a typical activated sludge treatment
system?
Aeration has the highest energy use. TVT BioReactors and Venturi Bio-Jet systems can
significantly reduce energy costs in providing oxygen to microbes.
SOURCE: Water and Wastewater Energy Best Practice Guidebook. Focus on Energy. Science Applications International Corporation, 2009.
How much food can bugs eat in a day in a TVT-BioReactor System?
Depending on the physical plant, flows, BOD loading, etc. in excess of 5,000 pounds BOD/day.