Water & Wastewater Asia Sep/Oct 2015 - page 36

September October 2015
Thinking Cap
34
Water & Wastewater Asia
Low Energy
Algae-Facilitated Biological
Wastewater Treatment
By
Adi Avni,
Senior Process Engineer, Aquanos Ltd.
Jonathan Liberzon,
Technology Development Manager, Aquanos Ltd.
Udi Leshem,
CEO, Aquanos Ltd.
hile numerous advances in biological wastewater
treatment have accrued in the past few decades,
the majority of Wastewater Treatment Plants
(WWTPs) worldwide continue to use essentially the same
activated sludge process that was discovered by Arden and
Lockett in 1913
[1,2]
. This process’ remarkable staying power
can be credited to its ability to effectively reduce both organic
compounds and nitrogen in a variety of wastewaters
achieving high effluent qualities, as well as its time-tested
reliability, worldwide applicability and relatively small land
footprint.
Despite this impressive track record, a changing world
is illuminating certain drawbacks of this process. Rising
energy prices and recognition of the detrimental effects
of carbon emission from energy production have brought
attention to water and wastewater supply and treatment
high energy requirements, which account for roughly
4% of the entire national energy consumption in the US
alone
[3]
. Land application of manures and other agricultural/
industrial wastes have led to problematic phosphorus
loading into natural waters
[4]
; and subsequent efforts to
find an appropriate solution for removing phosphorus from
wastewater have been disappointed by activated sludge’s
poor phosphorus reduction capabilities
[5]
. At the same time,
the saturation of farmlands with nutrient-rich agricultural
wastes have severely limited the disposal options for sludge
solids, thereby increasing the price and complexity of sludge
handling for activated sludge facilities, with sludge treatment
facilities account for third of the total WWTPs CAPEX and
50% of municipal WWTPs OPEX
[6]
.
In the 1950s, W.J. Oswald, a pioneering phycologist,
developed an alternative biological treatment process that
relays on photosynthetic oxygenation by algae for bacterial
decomposition of wastewater
[7]
. Unlike the activated sludge
process, which requires constant aeration to supply bacterial
communities with sufficient oxygen to break down organic
matter, algae ponds produce enough dissolved oxygen
through photosynthesis alone to completely oxidise all
dissolved organics
[8]
. Also, while activated sludge cannot
significantly reduce phosphorus levels in wastewater, algae
assimilate phosphorus during photosynthesis, and are able
to reduce phosphate concentrations below even the most
stringent discharge limits
[9,10]
. Finally, due to higher nutrient
contents and better physical characteristics, algae biosolids
are significantly more commercially desirable than bacterial
sludge, and may be converted into profitable coproducts
rather than nuisance wastes
[11,12]
.
The main problems with algae pond systems have always
been primarily twofold: large land footprints and difficulty in
algae-water separation
[14-16]
. Since Oswald’s time, significant
progress was made in improving the ability to separate
algae from effluent waters cost-effectively by different
means including the development of settleable stable algae-
bacterial flocs and new algae-appropriate coagulants and
flocculants which allow for traditional settling and coarse
filtration technologies
[18]
as well as dissolved air flotation
technologies
[19]
in biomass separation, resulting in dramatic
improvements in solids separation.
While solids separation techniques continue to improve,
almost no significant headway has been made in reducing
the cost-prohibitive land footprints necessary for algal
wastewater treatment. Algal based WWTPs generally
require longer retention times and high surface area
to volume ratios in comparison to traditional WWTPs.
These requirements are derived from the need of sunlight
availability for algae photosynthesis. In order to reach
a certain effluent quality, a certain amount of bacterial
biomass is required. When the biomass concentration
is too high it prevents light from penetrating the
water and thus hindering algae growth and oxygen
production. In order to enable efficient algae-bacteria
symbiosis, a low concentration of biomass is retained
and long retention times are employed, resulting in larger
footprints.
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