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9th European Waste Water Management Conference

12-13 October 2015, Manchester, UK

REAL CASE STUDY OF AN ENERGY EFFICIENT ADVANCED AERATION CONTROL

Abstract

The high power consumption associated with a sewage treatment works (STW) has a significant impact on

its operating costs and, no less importantly, on its sustainability measured in terms of its carbon footprint.

Reducing power consumption is therefore a priority. This article describes the successful measures taken

to reduce energy consumption at the Baix Llobregat STW using the advanced control system platform, atl,

from SISLtech. A reduction of 22.5% was achieved in the study year compared with the previous year,

without affecting the quality of the effluent, while continuing to comply with current legislation on water

discharges and reclamation and ensuring the treatment process equipment functioned correctly. The

energy savings in the aeration operations alone amounted to around 35.5%. The measures described were

implemented with the prior approval of the Catalan Energy Institute (ICAEN), which granted a subsidy

covering 40% of the total cost of the investment.

Key words

STW, water treatment, water reclamation, energy efficiency, carbon footprint, bioreactor, aeration, control

system

Introduction

The Baix Llobregat STW (Figure 1) is located to the south of Barcelona and serves a population equivalent

of 1,700,000 with a treatment flow rate of 315,000 m3/day. It also has a water reclamation plant with water

quality criteria that meet the requirements of Royal Decree 1620/2007 (Spanish legislation for water

reclamation) and a thermal sludge drying installation connected to a CHP plant with natural gas and biogas

engines and installed capacity of 10MW.

The STW discharges into the Mediterranean Sea via a 3.5 km marine outfall. The treated wastewater is

expelled at a depth of 60 metres through a diffuser system with duck-bill valves, compliant with the limits

set out in Royal Decree 509/1996 implementing Directive 91/271/EEC. STW effluent also feeds the

reclamation plant for water to be used in agricultural irrigation and for environmental, urban and industrial

purposes. For water destined for environmental uses, which will be used in sensitive areas as defined by

the EC Directive, it is necessary to reduce the nitrogen and phosphorus content. This process is carried out

during the biological treatment stage at the STW prior to feeding the water through to the water reclamation

plant.



Figure 1: Aerial view of the Baix Llobregat WWTP

Figure 2: Power consumption by biological treatment turbofans in Baix Llobregat WWTP for

different line configurations



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Organised by Aqua Enviro Limited

The STW has 12 plug flow bioreactors, split into two treatment lines with six reactors each. The

number of reactors operating on each line can be changed to allow the line to operate in BOD

removal mode or nutrient removal mode.

Depending on the treatment applied the configuration of the reactors can be Anoxic – Anaerobic –

Aerobic or Anaerobic – Aerobic.

The higher O2 requirement for the nutrient removal process is reflected in Figure 2, showing average

energy consumption by the turboblower section in three different operating scenarios:

L1 and L2 in conventional BOD removal configuration (green columns in chart).

L1 and L2 in nutrient removal configuration (blue columns).

L1 in conventional and L2 in nutrient removal configuration (grey column).

The air needed for the aerobic processes, carbon removal and nitrification is supplied using grids of

fine bubble diffusers in each tank. The air to the reactors is supplied by five blowers with a unit

capacity of 1200 kW.

Modification of the aeration blower control system

Initially, the air to be supplied to each of the 5 aerobic tanks in the 12 reactors was determined using

two automatic mechanisms based on PID logic:

- Pressure controller in the main air tube common to all 12 reactors

- Two dissolved oxygen controllers in each reactor (in the first and last chambers).

There was no automatic regulator in the middle two aerobic chambers in each reactor, which were

manually adjusted.

The control system had the following independent operating modes:

- Pressure set point in main air pipe, adjustable hourly, 24 hours a day.

- Regulation of air supply valves to first and last chambers of each reactor, in accordance with

the concentration of dissolved oxygen in each chamber.

- Regulation of 02 in the first chamber of each bioreactor in accordance with ammonium levels

at the secondary decanting outlet.

Figure 3: Turbofan room at Baiz Llbregat WWTP, 5 units

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The key element of the aeration system comprises five turboblowers (Figure 3) with the following

features:

Brand: HV TURBO

Model: KA66S

Flow (m3/h) : Min 22,381 - Max 49,736

Speed (rpm) : 7224

The motorised air valves that regulate the air entering each chamber equipped with oximeters (two in

each reactor x twelve reactors) are lens type valves equipped with an AUMATIC AC01.1 control unit

with Profibus DP and position transmitter.

The Energy Efficiency and Savings Plan initiated by EMSSA (Municipal Sewer and Wastewater

treatment Company) in 2009 prioritised replacing the PID controller on the air supply to the

bioreactors with an advanced controller which would reduce power costs both in terms of

consumption and the tariff paid.

Figure 4: The Energy Management System (EMS) installed in the Baix Llobregat STW has

two parallel operating platforms: an aeration control system (Sica) and a

control system for the removal of nutrients or carbon (Nutrien), the latter

determining the treatment set points for the former

An Energy Management System (EMS) from Sisltech was therefore installed in the Baix Llobregat

STW two parallel operating platforms: an aeration control system (Sica) and a control system for the

removal of nutrients or carbon (Nutrien) which determines the treatment set points for the Sica

system. These platforms are independent from each other and from the two main STW lines. They

therefore have a range of working configurations.

Figure 4 shows the flows for the EMS installed in the Baix Llobregat STW. The control and data

management modules contain the Sica and Nutrien platforms, which modify set points and at the






same time monitor the system. The decision support module collects and stores data from the line

sensors, while the energy management module records operating data from the STW equipment. The

platform thus receives and processes data from the STW, helping the manager to take decisions

which are then built back into the system.

The Sica aeration control system acts on the production and regulation of air based on previously

established oxygen set points, either through the parallel control system or through the set points

established by the manager. As previously detailed, in each bioreactor there are two devices

continuously measuring oxygen (Hach Lange, LDO model), one in the first and one in the last

chamber of the aerobic area, causing the valve regulating the supply of air to each chamber to move

so the flow complies with the established set point. Each time the valve moves there is a change in

pressure in the air pipe. This information is used to determine whether the system needs more or less

air. If more air is needed, this may involve increasing the output of a turboblower or starting up a new

turboblower. The valves on the middle chambers, which do not have oxygen meters, are controlled

using estimated values based on valves 1 and 4.

Based on the premise that every movement in the valves results in a change in the pressure, and this

is directly linked to the demand for air in the system and therefore the power consumption of the

turboblowers, an MOV (Most Open Valve) strategy is applied in some of the sections with higher

oxygen demand. This involves leaving the valves to some of the bioreactors completely open,

significantly reducing the number of operations needed to regulate the air supply along the line and

reducing pressure loss in the installation, thereby saving energy.

There are two potential operating scenarios: the conventional system (carbon removal) and the

nutrient removal system. Both are controlled by the Nutrien platform. In the carbon removal system,

the oxygen set points are established in accordance with the turbidity readings at the secondary

treatment outlet, which is continuously monitored using a Hach Lange Solitax turbidimeter. These set

points move between an upper and lower limit set by the manager. In the nutrient removal system, as

there are no daily limits for the removal of nitrogen and phosphorus defined in current legislation, or

for ammonium, but only average values for nitrogen, two configurations are possible:

- Real-time control: establishing set points based on the ammonium readings at the reactor

outlet. These set points move between a lower and upper limit set by the manager.

- Tariff-based control system: the system instantaneously adjusts the desired ammonium levels

in accordance with the price per kW/h. The concentration of ammonium in the composite

sample must be kept below the upper limit set by the manager. In this case, the off-peak

hours when electricity is cheapest are used to reduce ammonium concentration levels as

much as possible, supplying large quantities of air, while at the more expensive, peak times

the system is less strict, allowing ammonium concentrations to go higher.






Implementing the EMS at the Baix Llobregat STW involved the installation of energy meters in every

section of the plant, divided into the specific areas shown in Figure 5. An external energy audit was

carried out at the treatment plant in 2010. This can be taken as the starting point for analysing the

results obtained for consumption in the biological treatment processes, as shown in Figure 6.

In 2010 power consumed by the aeration processes represented 26% of the total consumption at the

plant, so any system to reduce air demand in the bioreactors and thereby save energy in this section

would have a significant impact on the plant’s overall consumption. Currently, and after the

implementation of the EMS, energy consumption for the turboblowers represents 23% of the total

plant consumption, indicating that the system has resulted in reducing energy costs.

It must be borne in mind that the accurate measurement of ammonium levels at the bioreactor outlet

is key to implementing a system to optimise power consumption in biological treatment processes. An

ammonium/nitrate combination sensor is installed in every bioreactor at the Baix Llobregat STW

(Hach Lange AN – ISE SC Ammonium - Nitrate Sensor), operated via six 2-channel SC 200






controllers, one for every two bioreactors, with six Profibus modules for each SC 200 controller. A

server to control the system is also installed. The ammonium and nitrate concentration data is sent to

the plant’s SCADA program (Simatic WinCC, version 6.0 SP3a) and a platform

connection/disconnection system (Figure 7) allows the user to visualise the data at any moment in the

following operating modes: Sica, Nutrien or PLC (conventional operating mode based on PID

controllers)

Figure 7: Connection/disconnection for interface for Sica and Nutrien platforms

The Baix Llobregat STW operates with one line in nutrient removal mode and the other in carbon

removal mode. The 12 bioreactors are grouped in 2 lines of 6 based on the A2/0 system, i.e., three

anaerobic chambers (800m3), three anoxic chambers (2,300m3), an optional chamber (700m3) and

four final aerobic chambers (6500m3).

The nutrient removal line operates with 6 bioreactors, with an average daily flow of 157,500m3. In

addition to external recirculation (input from the secondary decanter together with the waste water),

there is an internal recirculation current (bringing mixed liquor from the end of the aerobic section of

the bioreactor to the start of the anoxic section) which increases the concentration of nitrates in the

anoxic section of the bioreactor. This internal recirculation is also controlled from the EMS platform,

based on the nitrate readings at the reactor outlet (see Figure 8).

In addition to optimising power consumption by the internal recirculation pumps, installing combined

ammonium and nitrate sensors means a hybrid operating mode can be used. The hybrid mode

involves combining prolonged aeration of the system with cycles of minimal aeration when low

ammonium levels are detected, i.e. when they fall below the limits set by the manager, at which point

aeration in the reactor is reduced to just that needed to prevent sedimentation.






Figure 8: Internal recirculation pump data based on nitrate readings for a typical day

Figure 9: Main control variables in a bioreactor. Graph for the reactor with nutrient

removal in hybrid aeration mode. Data for same typical day as that shown in

figure 8






Figure 10: Main control variables in a bioreactor with carbon removal

During this cycle, microorganisms are forced to use the molecular oxygen in the nitrate to remove

organic matter, reducing the concentration thereof. This reduces the performance of the recirculation

pump and increases ammonium levels. Figure 9 shows this working in one bioreactor. Oxygen

consumption is thus reduced, thereby bringing down power consumption. The minimum aeration cycle

times and intervals and the ammonium and/or nitrate limits are set for the Baix Llobregat plant in order

to comply with current legislation while minimising power consumption.

The conventional line operates with 5 reactors (the sixth reactor is kept in reserve), with an average

daily flow of 157,500m3. There is no internal recirculation current and aeration is regulated on the

basis of the turbidity of the secondary treatment output (Figure 10).

The EMS came into operation in November 2011. This report therefore describes the results over

twelve months from that date. As mentioned previously, there are two independent treatment lines at

the Baix Llobregat STW. The Nutrien and Sica systems are activated on both lines, but using different

parameters to regulate air flow.

Ammonium is the limiting factor on the nutrient removal line. The removal of ammonium and nitrates

is continuously monitored by sensors at the outlet of each bioreactor. Aeration is strictly controlled on

the basis of these readings, preventing unnecessary excess aeration in the reactors. This line can

also operate in hybrid mode, allowing the minimum aeration cycles to be personalised, increasing

both the frequency and duration of shutdowns. Turbidity is the determining factor for the air supply set

points in the carbon removal line.

In both cases, whether the determining factor is ammonium or turbidity, it could be said that the main

difference compared with conventional operating systems is the use of automated dynamic aeration

set points. With a PID controller, there is a time lag between the measurements and the manager’s

reaction. This is not an issue with the EMS, as the system itself takes decisions quickly and

automatically.






Table 1 shows the quality parameters for each line during different operating periods at the Baix

Llobregat STW. Period 1 refers to a 12 month period prior to the implementation of the EMS and

period 2 refers to the study year.

There are no significant changes between the two periods in the quality indicators for the nutrient

removal line. Since the implementation of the EMS, the quality of the treated output has remained

stable or even improved for some parameters, while power consumption has dropped.

Table 1: Average quality readings for each period and each treatment line.

Parameter

Period 1* Period 2**

Line with nutrient

removal

Line with carbon

removal

Line with nutrient

removal

Line with carbon

removal

DBO5 (mg O2 /L) 6.42 8.84 4.82 9.64

TSS (mg /L) 16.67 19.92 13.36 22.82

TURBIDITY (NTU) 8.04 10.6 6.66 13.29

COD (mg O2 /L) 42.17 47.42 31.18 51.55

N-NH4 (mg /L) 1.4 15 1.57 32.89

NT (mg/L) 6.9 20.17 6.93 42.65

Note: *: Period 1: from November 2010 to October 2011; **: Period 2: from November 2011 to October 2012

In the case of the conventional treatment line, several indicators rose in period 2, but all the levels

remained within the legal limits, so excess power consumption was cut.

There are certain operational parameters that must be controlled. Turbidity at the secondary treatment

outlet is the key measure for the conventional treatment line as it is what drives the need for more air

and thus higher power consumption. This is a very robust and reliable measure though it may be

affected by dirt on the sensor optics. This is not an issue with proper maintenance, including daily

cleaning and preventive maintenance routines.

For the nutrient removal line, the ammonium/nitrate sensors are clearly the key to successfully saving

energy. The data from these sensors enable the system to decide whether or not to activate minimal

aeration cycles, increase the flow rate of the recirculation pumps, or supply more air to one or more

reactor. Unlike the turbidity sensors, these are extremely delicate instruments that must be fully

monitored, checked and calibrated by the system supervisor. The oxygen sensors in the reactors are

equally important and also require frequent cleaning and maintenance. It is therefore vital that there is

a preventive and predictive maintenance plan in place for every stage of operations.

The implementation of the control system has produced significant results, reducing energy

consumption for the supply of air to the biological system by 35%, consumption per cubic metre

treated by 33%, consumption per kg of COD removed by 43% and consumption per kg of nitrogen

removed by 21%.






Table 2: Real consumption in aeration process during the study period

Comparative study of energy efficiency

The real power monthly consumption figures for the aeration process at the Baix Llobregat STW since

the implementation of the EMS are shown in Table 2.

The effluent load was very similar in both years of the study, both in terms of organic material and

total nitrogen, and therefore had little effect on the study results. The energy cost reductions in the

aeration process following the implementation of the EMS as shown in the table above can therefore

be taken to be correct. These savings vary between 12.7% in May to 39.4% in August 2012, so the

overall savings are very significant.

Parameter

2010 2011 2012

% Reduction

KWh / month KWh / month KWh / month

January 1,197,820 877,168 26.8

February 1,063,330 771,070 27.5

March 1,081,611 772,488 28.6

April 1,149,159 799,536 30.4

May 1,034,486 902,663 12.7

June 1,097,430 818,078 25.5

July 1,095,030 758,946 30.7

August 991,415 725,502 26.8

September 1,134,358 687,145 39.4

October 1,113,542

November 1,123,082 896,299 20.2

December 1,123,652 926,099 17.6






Conclusions

The aeration control system implemented in the bioreactors has produced the following results:

- A 35% reduction in energy consumption related to the supply of air to the biological system,

while ensuring compliance at all times

- A 33% reduction in consumption per cubic metre treated (from 0.133 to 0.09 kWh/m3) during

a period when total treated flows fell by just 3.7%.

- A 43% reduction in consumption per kg of COD removed in the biological treatment process

(from 0.42 to 0.24 kWh/kg).

- A 21% reduction in consumption per kg of nitrogen removed (from 3 to 2.3 kWh/kg).

These effects have reduced both the operating costs of the Baix Llobregat STW (Figure 11) and its

carbon footprint, helping to make this large sewage management infrastructure more sustainable.

This is vital for the preservation of the environment in the Barcelona Metropolitan Area.

References

Ortega, E. (2009). Biological treatment of activated sludges [Tratamientos biológicos de fangos

activados]. CEDEX Centre for Hydrographic Studies. Course: Waste water treatment and the

operation of treatment plants. Spanish Ministry of Environment and Rural and Marine Affairs.

Baeza, JA; Gabriel, D.; Lafuente, J. (2002). Improving the nitrogen removal efficiency of an A2/0

based WWTP by using an on-line knowledge based expert system. Water Research, no. 6, pp. 2.109-

2.123

Yong, M.; Yong Zhen, P; Xio-lian, W,Shuying, W (2006). Intelligent control aeration and external

carbon addition for nitrogen removal. Environmental Modelling & Software, no. 21, pp. 821-828.


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