Title: SUSTAINABLE WASTEWATER TREATMENT
- WAYS TO ACHIEVE ENERGY NEUTRAL
Key Subject Area: Wastewater Treatment
Ms S W HO
Ir K K CHEUNG
Ir W C FUNG
Engineering Graduate
Engineer
Senior Engineer
Electrical and Mechanical Projects,
Electrical and Mechanical Projects,
Electrical and Mechanical Projects,
Drainage Services Department,
Drainage Services Department,
Drainage Services Department,
The Government of the Hong Kong
Special Administrative Region
The Government of the Hong Kong
Special Administrative Region
The Government of the Hong Kong
Special Administrative Region
Room 4413A, 44/F,
Room 4413A, 44/F,
Room 4413A, 44/F,
Revenue Tower, Wan Chai
Revenue Tower, Wan Chai
Revenue Tower, Wan Chai
2594 7327
2594 7338
2594 7322
[email protected]
[email protected]
[email protected]
Word Count: 5,789
SUSTAINABLE WASTEWATER TREATMENT
- WAYS TO ACHIEVE ENERGY NEUTRAL
ABSTRACT
Increasing emission of greenhouse gases intensifies global warming. To tackle the problem,
major economies in the world need to implement policy on energy saving. The wastewater
treatment industry, being one of the energy consumers, has developed various technologies to
improve energy efficiency, and to use renewable energy with an aim of achieving energy
neutral in the wastewater treatment process.
Literature review found that there is a series of energy saving method which can reduce the
electricity consumption by 20% as a whole. Majority of the energy saving comes from
energy conservation measures on pumping and aeration systems.
In wastewater treatment works, two kinds of renewable energy are usually available; namely,
solar energy and biogas. Recent development of photovoltaic cells makes it a more practical
and financially viable energy source which may cover 40% of electricity consumption of a
wastewater treatment works.
Biogas, a by-product generated from anaerobic digestion
process in a wastewater treatment works can also generate 40% of electricity required. To
achieve energy neutrality, the remaining electricity consumption can be generated through codigestion of wastewater sludge and organic fraction in municipal solid waste.
Keywords: Sustainable development; wastewater treatment; energy neutral; energy saving;
energy recovery; anaerobic digestion; co-digestion; sludge pre-treatment
2
INTRODUCTION
Greenhouse effect is a process by which thermal radiation from the Earth’s surface is
absorbed by atmospheric greenhouse gases (GHG), and is re-radiated in all directions. Since
part of this re-radiation is back towards the Earth’s surface, it results in an increase of the
surface temperature on Earth. It is this effect to keep the Earth warm. However, the effect,
when being intensified by increase of GHG emissions due to human activities, causing global
warming - an unequivocal and continuing rise in the average temperature of Earth’s climate
system. In 2013, the International Panel on Climate Change (IPCC) stated that the largest
driver of global warming is carbon dioxide (CO2) emissions from fossil fuel combustion,
cement production, and land use changes such as deforestation.[1] IPCC had stated that
“Human influence has been detected in warming of the atmosphere and the ocean, in changes
in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in
changes in some climate extremes”.[1]
We need to take actions to tackle global warming.
The United Nations Framework
Convention on Climate Change (UNFCCC), which had 196 parties including all United
Nations member states and the European Union, produced an agreement in 2010 that the
Parties should take urgent action to reduce GHG emissions to meet a goal of limiting global
warming to 2°C above pre-industrial temperatures.
In 2012, China also set new GHG
emission reduction goals in its newest 12th Five-year Plan (「十二‧五」規劃) to reduce
carbon intensity by 17% by 2015, and called for pilot programs that aim to promote a lowcarbon economy.[2]
With a view to reduce GHG emissions, the Chief Executive of Hong Kong had announced a
target in his Policy Address 2007/08 on reducing the energy intensity at least 25% by 2030
(with 2005 as the base year).[3]
These include promoting use of cleaner energy and
3
renewable energy, improving energy efficiency and energy conservation, encouraging
greening and raising public awareness.[3]
In response to the mission in reducing GHG emissions, the worldwide wastewater treatment
industry had paid tremendous efforts on it, through sustainable development of technologies
to reduce the net electricity consumption in the course of wastewater treatment. Energy
content analysis in some research studies indicated that the energy available in raw
wastewater exceeds the energy requirements of the wastewater treatment facility. Wastewater
contains thermal energy, hydraulic energy and calorific energy. The calorific energy of
wastewater is the energy content stored in the various organic chemicals. Its strength is
typically expressed as a chemical oxygen demand (COD) in milligrams per litre (mg/L).
Laboratory experiments using bomb calorimetry method found that wastewater with COD in
the range of 250 to 800mg/L (which is common for domestic wastewater) contains calorific
energy of about 1.5 kilo-watt hour per cubic metres (kWh/m3).[4] In theory, if all chemical
energy of the organic matters could be transformed into electricity, it should be more than
sufficient to meet the demand of a whole wastewater treatment works. The question is –
How to do it? Unfortunately, with our available technologies it seems that we cannot retrieve
100% of the calorific energy from wastewater. But, we still can collect and make use a
considerable amount of it. When playing together with other energy reduction practices, an
energy neutral wastewater treatment works, a plant which producing sufficient energy to drive
itself, is no longer an impossible thing.
4
ENERGY NEUTRAL
To build an energy neutral wastewater treatment works, we start with a ‘net energy balance’
analysis, whereby energy needs are balanced by energy supplied. To do so, we have to
identify energy needs which can be reduced, and find opportunities to generate or recover
energy to supply the remaining treatment needs.
ENERGY SAVING METHODS
Water Environment Research Foundation (WERF), under the Operations Optimization
research program, had developed tools and conducted research to promote energy efficient
best practices. One of the largest set of case studies on energy efficiency and production in
the wastewater sector was compiled by the Global Water Research Coalition (GWRC).
Supported by WERF and other international research organizations, GWRC prepared a
compendium of best practices globally. WERF compiled the energy savings achieved from
energy efficiency measures in North America and supported the global compendium.
A
summary of potential savings through of Best Practices is shown in Table 1. It shows the
potential energy savings available from switching to energy efficient practices.
Energy Conservation Measure
Treatment Stage
Energy Saving Range (%)
Aeration system optimization
Secondary treatment
15-38%
Addition of pre-anoxic zone
Secondary treatment
4-15%
Flexible sequencing of aeration
Secondary treatment
8-22%
Table 1. Information from WERF, 2011 (Source: Water Environment Research Foundation
[4]).
5
Researchers also advised that approximately 60% of the electricity used at secondary
wastewater treatment works globally is for aeration.[4] The figure would not be the same for
different place, scale or type of wastewater treatment works. It also depends on the operation
practice. For a typical secondary wastewater treatment works in Hong Kong (Figure 1), the
portion of electricity used for aeration was under control to about 33%, but still significant.
Therefore, saving in electricity by applying the energy conservation measures for secondary
treatment (Table 1) might be vast, and the potential is quite high.
Figure 1.
Distribution of Energy Usage in a typical wastewater treatment works with
treatment capacity of approximate 100,000 m3/day.
For secondary treatment, aeration system is the core part while blowers are an integral piece
of the aeration system. There are different designs of blowers, but all consist of lobes,
impellers, or screws mounted on one or more rotating shafts powered by a motor. The
working principle is when the shaft turns, the blower pulls in outside air and forces it through
distribution pipes into aeration basins at a pressure. The energy consumption of blowers is a
function of air flow rate, discharge pressure, and equipment efficiency. Blower efficiency
varies with flow rate, pressure, speed, inlet conditions and actual design. There were new
6
designs of blower with an aim to increase the efficiency. High‐speed gearless, or sometimes
called “Turbo”, blowers adopted advanced bearing design, i.e. air bearing or magnetic bearing,
to operate at higher speeds with less energy input compared to multistage and positive
displacement blowers. An air turbo blower is illustrated in Figure 2. When the shaft rotates at
high speed, an air film is formed between the impeller shaft and its bearings so that “friction
free” floating of the shaft can be achieved. Figure 3 shows a magnetic bearing design that the
impeller shaft is magnetically levitated for the provision of “friction free” floating of the shaft.
The friction free bearing design coupled with high efficiency motors contributes to a claimed
10% to 20% higher energy efficiency than conventional multi‐stage centrifugal or positive
displacement equipment.[5] This single stage blower, like the one in Shek Wu Hui Sewage
Treatment Works, has similar efficiency as fair film or magnetic bearing blower.
Figure 2. Example of High‐Speed Turbo Blower with Air Bearings (HSI) (Source: United
States Environmental Protection Agency [5]).
7
Figure 3. Example of High‐Speed Turbo Blower with Magnetic Bearings (Atlas Copco).
(Source: United States Environmental Protection Agency [5]).
Energy efficiency of an aeration system also depends on key factors including diffuser flux
rate, oxygen transfer rate, oxygen transfer efficiency (OTE) and more importantly, mixed
liquor dissolved oxygen (DO) concentration. The normal DO concentration for a healthy
activated sludge biological reactor usually ranges from 1 to 2 mg/L. It is critical to control
DO because both under-aeration and over-aeration have detrimental effects. Under-aeration
can lead to underperformance of the activated sludge process producing poor quality effluent.
On the other hand, when over-aerated, operational problems such as poor sludge settling and
negative impacts on the anoxic zone have been reported, in addition to wasting energy. As
such, a key point to save energy successfully is to have good control over DO levels so that
the aeration system supplies only what is needed. To do so, there are advances in dissolved
oxygen (DO) control strategies. Some examples such as Integrated Air Flow Control®,
intermittent aeration, and OPTIMaster™ are discussed here.
Integrated Air Flow Control® is a proprietary aeration control system which eliminates the
pressure control loop common in many automatic DO control systems. Particularly in smaller
systems, the pressure control loop can cause hunting as the control system attempts to adjust
8
air flow and pressure in response to changes in the process and ambient air conditions.
Implementation of the control system in the Narragansett Bay (Rhode Island) Commission’s
Bucklin Point facility in 2005 had provided the plant’s Modified Ludzack Ettinger (MLE)
process with the required DO for an average reduction of 12% electricity consumption.[5]
The energy savings was a result of eliminating the need to constantly run the second of the
two plant’s blowers.
Intermittent aeration saves energy by reducing the number of operating hours of an aeration
system. Air flow to an aeration zone or cycling air flow between zones is stopped
momentarily with a DO concentration or time based control. The air flow is switched on at a
set high level while turned on again at a set low level. Nevertheless, it is not appropriate for
all facilities, especially those at or near capacity, and needs to be evaluated on a case-by-case
basis so as not to adversely impact the wastewater treatment process.
Although reducing DO in the aeration process effects energy savings (i.e., less DO lowers the
electricity consumption of the blowers), it often requires increasing sludge age (SRT) to
compensate for the deterioration in process performance. Increasing the sludge age in an
activated sludge process, however, can lead to an increase in the sludge settling volume index
(SVI), which can increase the plant’s effluent total suspended solids (TSS). To tackle this, a
proprietary algorithm (OPTIMaster™) had been developed to optimize aeration. Oxnard in
California of the United States implemented the OPTIMaster™ system in 2006 and reported a
reduction of 20% electricity consumption.[5]
As mentioned above, the efficiency of an aeration system also depends on the diffusers,
especially their type. Fine bubble diffuser had been implemented for a long time and now is
considered as a conventional type. Recent developments in membrane materials had led to
ultra‐fine bubble diffusers (Figure 4), which generate bubbles with an average diameter
9
between 0.2 and 1.0 millimetres. The primary advantage of ultra‐fine bubble diffusion was
claimed to improve OTE by 10% to 20%. Additionally, some composite materials used in the
manufacture of ultra‐fine bubble diffusers were claimed to be more resistant to fouling, to be
able to maintain the high OTE with reduced frequency of cleaning.
Figure 4. Parkson’s ultra‐fine bubble diffuser (left) and AeroStrip® Diffuser (right) (Source:
United States Environmental Protection Agency [5]).
Pumping is also an essential step for wastewater treatment, and the energy required is high
which contributes to more than 15% of total energy consumption of a typical wastewater
treatment works (Figure 1). Since static lift is usually a major component of the system head
of a wastewater treatment works, it is imperative to keep the static lift to the minimum in
order to reduce the energy demand. Another thing that can be done to cut the static lift is the
use of variable speed drives (VSD) to maintain constant or near constant water level in the
inlet well, and the water level in the well should be kept at the highest acceptable level. With
the use of VSD and level sensor, the pump speed is controlled to maintain constant water
level which should be approaching the highest acceptable level. Energy saving from one to
two metres in static lift would be quite significant (10% to 20%) since the required static lift
for incoming wastewater is usually just a few metres.
10
MORE AND MORE
Apart from saving energy in existing wastewater treatment works, there are emerging
wastewater treatment processes which have the potential to make the greatest shift in the path
to energy neutrality. A summary of some technologies is shown in Table 2 below.
Wastewater Treatment
Methods
Sulphate Reduction
Autotrophic Denitrification
and Nitrification Integrated
(SANI) process
The Salsnes Filter
Membrane Enhanced
Primary Treatment
(MEPT)
Anammox
Bioprocess Intelligent
Optimization System
(BIOS)
Brief Descriptions
It is a novel and integrated biological nitrogen removal
process for saline wastewater treatment. This technology had
demonstrated a high removal efficiency of organic pollutants
at very low sludge yield.
It removes high percentages of TSS and particulate biological
oxygen demand (BOD) in wastewater in order to relieve
primary treatment burden in a very small footprint, saving
major infrastructure investment and space. It claimed to
contribute energy saving to the whole wastewater treatment
works.
It maximizes bio-adsorption of organic pollutant and to
oxidize organic pollutant and ammonia efficiently through a
membrane bioreactor.
The liquid sidestreams removed from biosolids processing
and returned to the main wastewater treatment process are
extremely high in nitrogen loads which add considerably to
the energy demand in conventional systems. Reductions in
the load from these sidestreams have the potential to reduce
the energy demand of the secondary treatment system.
It is a proprietary control algorithm and on-line process
simulation program to optimize the operation of a MLE
biological nitrogen removal process (Figure 5). It provides a
continuous output of DO set-points for the process according
to total ammonium load entering the bioreactor. It was
claimed that BIOS control could minimize aeration energy
consumption.
11
Wastewater Treatment
Methods
Brief Descriptions
SymBio
Itt is a process uses online monitoring of nicotinamide adenine
dinucleotide (NADH) to determine changes in biological
demands. Based on the results, air flow to the bioreactor is
controlled
to
promote
mote
simultaneous
nitrification
nitrificationdenitrification of wastewater.. The manufacturer claimed a 25
to 30% of energy savings by adopting the technology.
Table 2. Advanced Wastewater Treatment Technologies.
Figure 5. Representation of the BIOS process (Source: United States Environmental
Protection Agency [5]).
Energy efficiency improvement is part of the process to reduce energy demand along the path
to a net energy neutral wastewater treatment works but cannot achieve that goal alone.
Previous discussions showed that a significant amount of energy consumed during
dur
the
wastewater treatment process can be saved by means of energy conservation measures and
advanced wastewater treatment technologies.
Nonetheless, energy neutral wastewater
treatment requires energy recovery, by promoting improvements in anaerobic digestion
di
and
further developing alternative processes to recover energy from wastewater.
wastewater
12
ENERGY RECOVERY
There are several types of technologies and opportunities to recover energy throughout the
wastewater treatment process. WERF had conducted a review in these areas and a summary
of energy recovery potential using established technologies is shown in Table 3.
Biosolids Technology
Anaerobic Digester (AD)
Biogas with boilers
AD Biogas with co-gen
engines
AD Biogas with micro
turbines
AD Biogas with turbines
AD Biogas with fuel cell
AD Biogas after WAS
pretreatment
AD Biogas with Codigestion
Table 3. Information from
% of Net Energy
Gap Reduction
Possible
13 – 57%
11 – 61%
5 – 38%
Other
Technology
Enhanced solids
removal
Anaerobic
primary
treatment
Heat recovery
% of Net Energy
Gap Reduction
Possible
10 – 71%
25 – 139%
13 – 49%
7 – 46%
6 – 42%
-2 – 60%
Hydraulic
0%
Ammonia as fuel -6 – 12%
Heat from
13 – 49%
centrate
2 – 128%
Microbial fuel
8 – 110%
cells
WERF, 2011. (Source: Water Environment Research Foundation
[4]).
The highest potential for energy recovery at wastewater treatment works is from biosolids.
Unprocessed biosolids typically contain calorific energy of 18,000 kJ/kg on a dry weight basis.
The potential for energy recovery from biosolids is a function of their composition,
specifically the relative proportions of inert material, biodegradable volatile solids, and nonreadily biodegradable volatile solids. There are established pathways for energy recovery
from biosolids such as anaerobic digestion and thermal conversion.
Anaerobic digestion process is widely used to convert the organic matters in wastewater
sludge to stabilized biomass. It encompasses complex interactions among bacteria and occurs
13
in phases: hydrolysis, acetogenesis, and methanogenesis (Figure 6). Hydrolysis is the first
step where complex organics are converted to soluble organics through extracellular enzymes.
Acetogenesis is the complex step where acid producing bacteria (acetogens) convert soluble
organics into volatile fatty acids, with acetic acid as the fully converted end point.
Methanogenesis is the next step where the volatile fatty acids are converted to methane, and
carbon dioxide.
In short, the process converts organic matters into biogas, primarily
composed of methane (60-65%) and CO2 (35-40), and water.
Food
(Organic
Matter in
Sludge)
Organic
Acids
Acid
Forming
Bacteria
CH
+
CO
Methane
Forming
Bacteria
st
4
2
nd
1 Stage
2 Stage
Figure 6. Anaerobic Digestion Process.
Biogas collected can then be converted to electricity and heat using onsite Combined Heat
and Power (CHP) generators. Heat can be recovered from the power generation units to heat
the digesters, or for other purposes in wastewater treatment works.
Anaerobic digestion, coupled with CHP generators for energy recovery, is considered as one
of the most mature and successful energy recovery approaches worldwide. Recent efforts in
the industry are to find ways in boosting up the production of biogas during anaerobic
digestion process. Technologies such as co-digestion and sludge pre-treatment are emerging
on the path.
14
CO-DIGESTION WITH FOOD WASTE
Co-digestion is the simultaneous digestion of a homogenous mixture of two or more
substrates. The most common situation is when a major amount of a main basic substrate (e.g.
wastewater sludge) is mixed and digested together with another single, or a variety of
additional substrates. The possible use of wastes as co-substrates in anaerobic digestion is
influenced and determined by European Union (EU) legislation (Figure 7) and/or National
legislation and technical guidelines on waste recovery. Landfilling of untreated organic waste
is gradually being reduced and in some of the European countries is completely prohibited.
Some of the legislation promotes co-digestion.
Figure 7. Overview on legislation and guidelines influencing organic waste recovery (Source:
Braun R., Wellinger A[6]).
Furthermore, due to increased biogas yields, the co-digestion of bio-wastes together with
municipal wastewater sludge in existing digesters can considerably reduce wastewater
treatment costs. Therefore, in some European countries, organic wastes are co-digested in
municipal wastewater sludge digesters. Some successful examples from sewage treatment
15
works have been reported in Denmark (e.g. Lemvik, Grindsted), Germany and Switzerland. [6]
In the United States, food waste is the second largest category of municipal solid waste sent to
landfills and over 30 million tonnes of food waste is sent to landfills each year. In the past
few years, there had been a movement in the United States to start adding food waste to the
anaerobic digesters already in wastewater treatment facilities. As energy prices continue to
climb and people look towards renewable energy generation and energy independence,
capturing the energy from food waste becomes more important.
At the East Bay Municipal Utility District’s (EBMUD) Main Wastewater Treatment Plant
(MWWTP) in California of the United States, food waste was co-digested with primary and
secondary municipal wastewater solids and other high-strength wastes since the year 2008 as
a study on anaerobic digestion of food waste. Bench-scale anaerobic digesters were fed only
food waste pulp from EBMUD’s food waste processing system (Figure 8).
Figure 8. EBMUD Food Waste Treatment Process (Source: Gray M.D., Suto P, Peck C[7]).
Food waste anaerobically digested at EBMUD’s MWWTP is collected from local restaurants,
grocery stores, and produce markets that source separate food waste. The collected food
waste is pre-processed by a local hauler, and then further processed at EBMUD’s MWWTP
16
into a food waste pulp, to reduce contaminants and prepare the food waste for optimal
digestion, prior to pumping to EBMUD’s anaerobic digesters. The digesters were operated at
both mesophilic and thermophilic temperatures, at 15-, 10-, and 5-day hydraulic retention
times (HRTs). In addition, anaerobic food waste digestion was compared with anaerobic
municipal wastewater solids digestion to demonstrate the benefits of food waste digestion at
wastewater treatment works.
Results of the study had demonstrated that food waste is more readily biodegradable than
municipal wastewater solids.[7] Anaerobic digestion of food waste can be achieved at a
shorter HRT (i.e. 5 or 10 days) than that for municipal wastewater solids (around 20 days in
various wastewater treatment works in Hong Kong). In other words, the feed rate of food
waste to anaerobic digesters can be 2 to 3 times to that of municipal wastewater solids. More
importantly, food waste has higher specific energy content than municipal wastewater solids.
Food waste digestion results in a nearly 3 times higher biogas production rate when compared
to that of municipal wastewater solids digestion.
According to “Monitoring of Solid Waste in Hong Kong – Waste Statistics for 2011”
published by the Environmental Protection Department of the Government of Hong Kong
Special Administrative Region, average daily quantity of food waste disposed in 2011 was
3,584 tonnes (Tables 4). If all the food waste is diverted to wastewater treatment works and
as an external energy source, is fed to anaerobic digesters and co-digested with wastewater
sludge, the amount of energy recovery via CHP will be very huge. The estimated amount of
electricity production should be more than sufficient to meet the electricity demand for
wastewater treatment.
17
Table 4 – Composition of Hong Kong’s municipal solid waste in 2011.
SLUDGE PRE-TREATMENT
Degradation of the organic sludge fraction by conventional anaerobic digestion is limited by
the hydrolysis step. Degrees of volatile solids degradation of 50% are rarely achieved. The
cause of this lies in the difficulty to access and degrade bacterial biomass of the waste
activated sludge.
There are ways to improve the degradation of volatile solids with a view to increase the
biogas production rate. Using thermal hydrolysis such as the Cambi™ process as a pretreatment to anaerobic digestion can increase the biological degradation of organic volatile
18
solids and biogas production considerably. It makes the sludge readily available for digestion
dissolving and decomposing the solids, while at the same time facilitating a higher degree of
separation of solid and liquid phase after digestion. Thermal hydrolysis was claimed to be
able to convert up to 62% of the volatile solids to biogas, in comparison to 30-50% in the
conventional anaerobic digestion process. [8]
In addition, it was claimed that by applying the high-power Ultrawaves™ ultrasound
technology, the limiting hydrolysis step can be overcome. The sonicated sludge biomass will
be more readily available for the subsequent biological enzymatic degradation process.
Figure 9. Disintegration of biomass by ultrasound (Source: ultrawaves.de[9]).
Ultrasound causes disintegration of the sludge floc structure and release of exo-enzymes even
with small energy inputs (Figure 9). This also creates more interface between the solid and
liquid phase and therefore facilitates the enzymatic attack of the active micro-organisms. A
higher energy input results in the breakdown of bacteria cells, causing the cell contents and
endo-enzymes to be released. These enzymes further accelerate the degradation process. The
entire digestion process is intensified and the organic fraction is further degraded.
An
important advantage from this is a significantly increased production of biogas and reduction
in the quantity of residual sludge to be disposed of.
As a result of the smaller quantity of
residual organic matter, the dewaterability of the digested sludge is also facilitated and
19
increased. It was claimed that in a wastewater treatment works with 100,000 population
equivalent, ultrasound achieves a 10% relative increase in anaerobic sludge degradation.[9]
THE SUN
When implementing the energy recovery technologies of activated sludge ultrasound pretreatment and followed by co-digestion with food waste in anaerobic digesters, we should be
able to generate sufficient electricity through CHP to operate a wastewater treatment works,
and achieve the target of energy neutral. It is a long way to apply all the energy conservative
measures and energy recovery methods in a wastewater treatment works. On the road to
energy neutral, there are other helpers – solar energy from the Sun.
Solar energy is the most abundant energy resource on earth. The solar energy that hits the
earth’s surface in one hour is about the same as the amount consumed by all human activities
in a year. Direct conversion of sunlight into electricity in Photovoltaic (PV) cells is one of the
three main solar active technologies, the two others being concentrating solar power (CSP)
and solar thermal collectors for heating and cooling (SHC). Today, PV provides only 0.1% of
total global electricity generation. However, PV is expanding very rapidly due to effective
supporting policies and recent dramatic cost reductions. PV is a commercially available and
reliable technology with a significant potential for long-term growth in nearly all world
regions. In the International Energy Agency (IEA) solar PV roadmap vision, PV is projected
to provide 5% of global electricity consumption in 2030, rising to 11% in 2050 (Graph 1), and
avoid 2.3 giga-tonnes (Gt) of CO2 emissions per year (Graph 2).[10]
20
Graph 1.
Global PV power generation and relative share of total electricity
generation.(Source: International Energy Agency[10]).
Graph 2.
Annual CO2 emissions avoided through PV(Source: International Energy
Agency[10]).
The basic building block of a PV system is the PV cell, which is a semiconductor device that
converts solar energy into direct-current (DC) electricity. PV cells are interconnected to form
a PV module, typically up to 50-200 Watts (W). The PV modules combined with a set of
additional application-dependent system components (e.g. inverters, batteries, electrical
components, and mounting systems), form a PV system. Commercial PV modules may be
21
divided into two broad categories: wafer based c-Si and thin films. Table 5 provides the
current efficiencies of different commercial PV modules.
Table 5. Current efficiencies of different PV technology commercial modules.
One essential criterion for the installation of PV system is the availability of significantly
large open areas because of its footprint requirement. In fact, apart from cost effectiveness,
this is always a critical factor that hinders the use of PV systems, especially in a very crowded
city where everyone has a very careful mind on the use of every piece of land. This, however,
may not be a problem in wastewater treatment works.
Wastewater treatment works, which needs to treat a vast amount of wastewater up to an
acceptable quality before discharge into the water body, always occupy large footprint.
Although not everywhere in a wastewater treatment works can be used for the installation of
PV modules, the top of sedimentation tanks and biological reactors, when covered, would
undoubtedly be adequate places. A large PV system, when connected to the grid of the power
supply company, will provide a portion of electricity to the wastewater treatment works, and
thus contributes to achieving the target of energy neutral.
22
IS IT POSSIBLE? LESSON LEARNED FROM OTHERS
Indeed, a wastewater treatment works can produce more electrical energy than it requires for
its operation. Located near Strass im Zillertal, the Strass wastewater treatment works serves
31 communities in the Achental and Zillertal valleys east of Innsbruck, Austria. It provides
wastewater treatment for a population that ranges from approximately 60,000 in the summer
to 250,000 during the winter tourist season, and has treatment requirements that include
organic and nitrogen removal.
The plant was commissioned in 1999 and successive
optimization efforts over the past decade have resulted in significant cost and resource
reduction. Efforts included active management of DO set points and conversion of the
aeration system from conventional fine bubble to ultra-high efficiency strip aeration. It also
employed swing zones that can be used in either aerobic or anoxic modes, alternating to
minimize air supply and energy requirements.
There was a 50% reduction in energy
consumption for sidestream treatment by implementing a novel sidestream nitrogen removal
system.[11]
23
A HYPOTHETICAL ENERGY NEUTRAL WASTEWATER TREATMENT WORKS
To illustrate the possibility to energy neutrality, the energy consumption and production of a
hypothetical secondary wastewater treatment works with treatment capacity of 100,000m3/day
(Figure 10) are tabulated in Table 6.
Without Energy Conservation Measures
With Energy Conservation Measures
Figure 10. Schematic flow diagram of a hypothetical secondary wastewater treatment works
with treatment capacity of 100,000m3/day.
24
System
Annual Electricity
Consumption without
Advanced Energy
Conservation Measures
(Giga-watt hour)
Annual Electricity
Consumption with Advanced
Energy Conservation
Measures
(Giga-watt hour)
12.801
10.35
2.01
4.19
6.59
1.613
2.154
6.59
(7.56)
(3.94)
(7.93)
(3.94)
(3.62)
(3.98)5
5.24
2.42
Electricity consumption
of the whole wastewater
treatment plant (a)
Pumping
Aeration
Other
Electricity production (b)
Solar Energy2
Biogas from Anaerobic
Digestion + CHP
Net Electricity
Requirement (c)= (a)-(b)
External Energy Source:
(5.24)
(2.42)
Anaerobic co-digestion Require to import 151tonnes of Require to import 70tonnes of
food waste per day
food waste per day
of food Waste + CHP (d)
Balance (c)-(d)
0.00 (Energy Neutral)
0.00 (Energy Neutral)
Notes:
1. Reference to a wastewater treatment works in Asia.
2. For a secondary wastewater treatment works of treatment capacity of 100,000m3/day, the
area available for the installation of photovoltaic panels is about 30,000m2. The actual
power output is estimated based on a land utilization rate of 50% and average energy
efficiency of 15%.
3. 20% energy saving comes from minimization of static lift, and the use of VSD.
4. Advanced energy conservation measures comprise the use of magnetic bearing high speed
turbo blower, ultra-fine bubble diffusion system, and smart control of DO. Each measure
contributes to a 20% of energy saving.
5. Advanced anaerobic digestion can boost up 10% energy production.
Table 6. Energy Balance Sheet for a hypothetical secondary wastewater treatment works with
treatment capacity of 100,000m3/day.
25
Percentage of energy saving for wastewater treatment by energy conservation measures
=
12.8 − 10.35
= ~20%
12.8
Percentage of energy
offset from solar energy
Percentage of energy offset
from biogas
Without advanced energy
3.94
=
= ~31%
conservation measures
12.8
with advanced energy
3.94
=
= ~38%
conservation measures
10.35
Table 7. Analysis on energy production from renewable energy.
3.62
= ~28%
10.35
3.98
=
= ~38%
10.35
=
As shown above, energy neutrality can be achieved by implementing energy conservation
measures for the pumping system, the aeration system and the anaerobic digestion system,
and energy production facilities from photovoltaic system and CHP. There is a net electricity
requirement which can be offset by importing a few tonnes of food waste, as an external
energy source, for anaerobic co-digestion to boost up the biogas generation rate. Analysis on
energy balance reveals that there will be only about 20% of energy saving even when all the
available advanced energy conservation measures are adopted, while about 80% of electricity
consumption can be covered by renewable energy (Table 7). Therefore, it is advisable to put
more effort on the utilization of renewable energy. To energy neutrality, only 2 to 5% of
daily food waste is required as a substrate to meet the remaining energy demand.
As
illustrated in the above hypothetical work, energy neutrality of a wastewater treatment plant
of treatment capacity of 100,000m3/day can save about 10 to 13 Giga-Watt hour (GWh)
electricity consumption a year, which is equivalent to an annual CO2 emission of 14,800 to
18,300 tonnes. It would take 644,332 trees to absorb this amount of carbon in one year. [12]
26
CONCLUSION
The global engine to combat greenhouse effect has been started for many years, and will
surely be kept running. Efforts had been paid by scientists and engineers on developing and
applying innovative and green methods in order to reduce carbon emission from different
kinds of human activities.
For the wastewater treatment industry, there are emerging
technologies available for minimizing the consumption of electricity in the course of water
pollution control and environmental protection.
To achieve energy neutral, we have two strategies. On one hand, the energy consumed must
be minimized. This can be done by adopting various energy conservation measures and
energy saving wastewater treatment processes. On the other hand, the energy stored in
wastewater should be retrieved and transformed to electricity for use. External source like
food waste can surely give us a hand on this purpose. More than enough, renewable energy
sources such as the solar energy from the Sun can do us a favour to provide the energy we
need. By adopting the strategies, an annual CO2 emission of 14,800 to 18,300 tonnes can be
saved. Otherwise, it would take 644,332 trees to absorb this amount of carbon in one year.
The road to energy neutral is straight forward, the direction is clear, and the destination is just
in front.
ACKNOWLEDGMENTS
First of all, the authors would like to take this opportunity to express their deep thanks to
Drainage Services Department for the participation in this paper. The first author is thankful
to co-authors, Mr. W.C. FUNG and Mr. K. K. CHEUNG, for their kind support that was
provided during the course of this work.
27
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