Case Study SA-RU
COMPARATIVE ANALYSIS BETWEEN WASTEWATER TREATMENT PONDS
SYSTEMS
Comparative Analysis between the Oxygenation Capacity (OC) and Oxygenation Efficiency (OE)
of the EBRU IAPS High Rate Algal Ponds(HRAP) and conventional aeration systems. (CAS)
Description of Process:
The introduction of oxygen into aHRAP is very different from that of conventional systems. Due to the nature of
the system, the water cannot be deoxygenated to enable a transfer coefficient to be determined. This is because
the oxygen is derived mainly by photosynthesis from the algal biomass and to a lesser extent from the surface
agitation caused by the paddlewheel. These separate figures have not been determined, but is worth
investigating in future research and applications.
The paddlewheel is designed to pump water, at a specific linear velocity, around the raceway. The action of the
paddle on the water causes a wave –like response at the water surface that moves around the raceway. This
turbulence results in the introduction of oxygen, and carbon dioxide, from the air. Again, these values are not
known and will require further application at the bench and in field.
Researchers, (Oswald 1988, 1990), have calculated the total OC of the pond and the installed power of the
paddlewheel to give an OE of 15 KgO2/KWh. This is a factor of 10 greater than the best mechanical aerators in
the MLSS. Thus, oxygenation capacity can be contrasted with mechanical aerators which rarely transfer oxygen
from air to water at more than ONE kg/kWh. Photosynthetic oxygenation is between 10-100 times more
efficient as all energy is solar derived.
Potential Interventions


As a result of the poor OE of mechanical aerators the DO in the MLSS is maintained at 2 mg/L and less to
conserve energy. In the HRAP a DO figure of three times saturation is not uncommon.
This, combined with the elevated pH levels, have a profound influence on the treatment process.
Range of potential savings
Previous South African case studies showed that technologies vary in effectiveness to provide uniform oxygen
absorption into sewage water. Typical Oxygen Transfer Rates for different aerator systems at sea level in clean
water are:





High speed Surface aerators
Low speed surface aerators
Coarse bubble aeration
Turbines or jets
Fine Bubble Aeration
0.9 to 1.3 kg O2 / kWh
1.5 to 2.1
0.6 to 1.5
1.2 to 1.8
3.6 to 4.8
Preliminary estimates by researchers at EBRU, Rhodes University for high rate algal ponds are 15 kg O 2/kWh,
based on the installed power of a paddlewheel. Further quantification of energy aspects were not included as
part of the study and will receive attention in future work.
Case Study SA-RU
Case Study SA-RU
COMPARATIVE ANALYSIS BETWEEN WASTEWATER TREATMENT PONDS
SYSTEMS
Case Study SA-RU
Ref
Response information, description and remarks
1
Location:
South Africa, Eastern Cape – various pilot sites for high rate algal pond
systems – this study reflects on the Grahamstown pond system.
2
Sector:
Sewage
3
Works Owner or Operator:
The pilot-plant is owned by the Makana Local Municipality and Rhodes
University who have incorporated the technology alongside the
Belmont Valley WWTW.
4
Size:
Different sizes and plant technologies are available, but usually 75
m3/d equivalent to 500 PE.
5
Energy Provider::
Power provision to all case sites is in form of electricity, provided
by the national electricity agency, ESKOM. Electricity tariffs average
out at R R0-35 to R 0-52 /kWh. This cost is high when considering
the low income base by many rural and semi-urban communities.
6
Process:
This case study does not consider a specific process or intervention,
but aims to highlight the gaps and considerations in the case selection
pertaining to energy interventions going forward.
7
Component:
This case references all parts pertaining to the high rate algal pond
system and its limited energy requirements.
8
Motivation for the case study:
The cost of electricity is set to double over 13 years starting 2008 in
South Africa. Small communities and rural municipalities will not be
able to afford energy intensive technologies and renewed focus is
required for less energy intensive systems.
9
Process/Plant changes:
Comparison of energy requirements between different types of
technologies as pertaining to its oxygen capacity and oxygen
efficiency.
10
Civil/Physical Changes:
No physical changes applicable, the system involves a biological
treatment with limited energy requirements.
11
Operational Changes:
Low skills base required to operate system.
12
Risks and Dependencies:
A major risk is that the national Department of Water Affairs imposes
high effluent standards, which makes simpler systems such as
pondspassed over when funding is considered, in favour of high
technology systems, even though high rate algae ponds produce an
affluent that is fully compliant.
13
Implementation:
The case study does not involve project implementation, but serves
to highlight energy issues on a comparative basis amongst
technology types, with specific focus on ponds systems.
14
Energy Efficiency gains:
Data available (i.e. 250Wh/ 150 m3 = 1,67Wh/m3)
15
Cost/Benefit analysis:
No detail provided. Work continuesby researchers at the University in
2012-13 to collect information. For energy use, already given in values
for kg O2/kWh (OE)
16
Project review:
A quantified basis from actual case studies is required going forward.
Documentation of case studies would present a major improvement.
17
Confidence grade:
High, in terms of energy-oxygenation requirement.
Medium, in terms of more research required to incorporate costenergy comparisons with other technologies.
Background to Case Study
The IAPS, at the Institute for Environmental Biotechnology, Rhodes University (EBRU) Experimental Field
Station was used as the experimental system for the study to be reported here. Although the Belmont Valley
IAPS was intended as a demonstration unit, it was sized to provide credible performance data and suitable
for engineering scale-up requirements. Design rationale and calculations were provided by Prof William
Oswald and Dr. Bailey Green, consulting as Oswald Green and were used as the basis for the conceptual plan
for the construction of the plant (Rose et al., 2002a).
The integrated algal ponding system plant at the Experimental Field Station, showing the primary facultative pond (top)
and high rate algal ponds, algal settling ponds and drying beds
The system was built to have the capacity to treat the liquid wastes of 500 person equivalents (PE). An
average water consumption and disposal per capita of approximately 150L.day-1 was assumed. Accordingly
the design flow was calculated at 75m3.day-1. With an ultimate Biochemical Oxygen Demand (BODult)
assumed to be 80g BODult per person per day, the organic loading to the system is 40kg.day-1.
Using an assumed conservative BOD loading to the HRAP from the PFP, the depth in the two HRAPs was set
at 30cm. The total volume of each pond is, therefore 150m3, with a surface area of 500m2. Using adjustable
overflow weirs into the anaerobic digester the hydraulic loading, and thus HRT, in the HRAP can be adjusted
to equal or less than influent flow (up to a maximum of 75m3.day-1 for the EBRU HRAP design capacity) for
experimental purposes, but was generally operated between 3 and 6 days. The algae floc is kept in
suspension in the HRAP by a paddle wheel that maintains a linear velocity of 30cm.sec-1 in the pond.
HRAP Energy considerations and theory
The ability of a system to
introduce oxygen into a body of
water/sludge is the OC of that
system. The energy required to
introduce the oxygen is the OE
of the system.
In general
aeration systems are usually
compared by their OE. This is
usually expressed as the amount
of oxygen introduced into the
water per unit of energy
(kgO2/kWh).
OE is also referred to as the
oxygen transfer rate (OTR) and
is a key factor in selecting aeration equipment for a specific application. Other factors that are equally
important are reliability, serviceability, capital cost, altitude above sea level and cost of operation and
maintenance.
Although there are many systems designed to aerate and mix wastewater, they vary in effectiveness of
providing uniform oxygen absorption into the sewage water.
Typical Oxygen Transfer Rates for different aerator systems at sea level in clean water are:





High speed Surface aerators
Low speed surface aerators
Coarse bubble aeration
Turbines or jets
Fine Bubble Aeration
0.9 to 1.3 kg O2 / kWh
1.5 to 2.1
0.6 to 1.5
1.2 to 1.8
3.6 to 4.8
Ref: Aeration study by University of California www.seas.ucla.edu.sentro/s/s6 Prof M K Stenstrom.
In a CAS the oxygen is introduced into the body of water by bringing it into contact with air. The oxygen is
diffused into the water by means of the interface between air and water. This diffusion rate follows Fick’s
Law. The quantity of oxygen that is introduced per unit time through the interface is determined by the
diffusion constant and concentration gradient. This is also known as the transfer coefficient. There are many
variables that influence the quantity of oxygen introduced. The OC depends on the characteristics of the
water being aerated, clean water, activated sludge, suspended solids, dissolved solids, surface active agents,
geometry of the aeration basin, type of aeration system used, etc.
From the above the OC can be calculated using either a Steady State or Unsteady State method.
Description of Process- Oxygen Transfer and the HRAP
The introduction of oxygen into the HRAP is very different from that described in the preamble above.
Due to the nature of the system the water cannot be deoxygenated to enable a transfer coefficient to be
determined. This is because the oxygen is derived mainly by photosynthesis fromthe algal biomass and to a
lesser extent from the surface agitation caused by the paddlewheel. These separate figures have not been
determined.
The paddlewheel is designed to pump the water, at a specific linear velocity, around the raceway. The action
of the paddles on the water causes a wave action at the surface of the water that moves around the
raceway. This turbulence must cause the introduction of oxygen, as well as carbon dioxide, from the air.
Unfortunately these values are not known.
Researchers, Oswald and others, have calculated the total OC of the pond and the installed power of the
paddlewheel to give an OE of 15 KgO2/kWh. This is a factor of 10 better than the best mechanical aerators in
the MLSS.
Schematic illustrating the process flow for the pilot IAPS designed, constructed and operational at the Belmont Valley
WWTW, Grahamstown. The system receives 80-100 m3 of raw sewage daily, screened for the removal of plastics, and a
grit or detritus channel (in duplicate - one operating, one cleaning). Effluent enters at the bottom of the AFP some 6 m
below water level. AFP=Advanced Facultative Pond; IPD=In-Pond Digester; HRP=High Rate Pond;
C/F=Coagulation/Flocculation; ASP=Algae Settling Pond; SB=Splitter Box.
Different configurations of the HRAP unit operation.
As a result of the poor OE of mechanical aerators the DO in the MLSS is maintained at 2 mg/l and less to
conserve energy. In the HRAP a DO figure of three times saturation is not uncommon. This, combined with
the elevated pH levels, have a profound influence on the treatment process.
Energy efficiency and cost considerations
Work is ongoing and focuses primarily on the treatment efficiency of the technology. Further work will
continue with increased emphasis on energy requirements, optimisation and efficiency.
Acknowledgement:



Prof Keith A. Cowen, Professor & Director, Institute for Environmental Biotechnology, Rhodes
University
Prof Peter Rose, Professor, Institute for Environmental Biotechnology, Rhodes University
Design rationale and calculations were provided by Prof William Oswald and Dr. Bailey Green,
consulting as Oswald Green

Rose et al., 2002a, Chapter 2, Methodology.