Comparative Evaluation of the Nitrogen Dioxide (NO2) absorbing capability of the AEROGATION Active Green Wall System and a Passive Green Wall System (Pilot).

Prof. John Dover The Green Wall Centre,

The Science Centre, Staffordshire University, Leek Rd, Stoke-on-Trent ST4 2DF

j.w.dover@staffs.ac.uk Tel:+44(0)1782 294611

FINAL REPORT 18/07/16

INTRODUCTION

The Green Wall Centre of Staffordshire University was commissioned by Armando Raish of Treebox Ltd and Mark Prescott of Agrosci Inc. to carry out a comparative study of the Aerogation Active living wall system (Figure 1) in reducing Nitrogen Dioxide (NO2) air pollution simultaneously with the system set up as a Passive living wall – i.e. with NO2-laden input air being passed over the vegetation of the living wall, but not through the root zone.

In the Aerogation Active system, air is pumped into the air purification unit (APU) where it picks-up moisture from wicks fed with water from enclosed troughs. The moist air is then introduced into the root zone of plants held in individual planters where microbiological communities can break-down pollutants (Figure 2). An example of a planted-up APU living wall can be found in Figure 3.

Figure 3. A planted-up APU

METHOD

The experimental rig design used in this comparative evaluation was the same for the Aerogation Active system as used in Dover (2016) (Figure 4).

Figure 4. The experimental rig used for the Aerogation Active unit. a) Nitrogen dioxide-laden air ( ) is introduced into a mixing chamber (c) from a fume cupboard; the gas is then passed through the rear of the rig via pipework which then (b) bifurcates and connects with the APUs pipework. NO2 sensors are positioned in the input mixing chamber (c) and suspended from the lowest row of planters in the (exhaust) polythene bag (d). The yellow arrow in (d) indicates the approximate position of the exhaust sensor inside the exhaust bag.

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NO2 sensor

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This design had to be modified for the Passive green wall (Figure 5). In this design, the NO2/air mixture was introduced into the top of the polythene chamber and the inlet to the rear of the Aerogation unit sealed off. Prior to this, the Aerogation unit’s water galleries were emptied of water by syphoning, and air pumped through the system for four days to dry-out any residual water.

Figure 5. Experimental rig modifications for the Passive green wall. (a) Nitrogen dioxide-laden air is introduced into the mixing chamber, as for the Active unit (airflow in red). However, on exiting the mixing chamber the pipework is connected to the top of the polythene bag (b) and the input to the APU pipework is sealed off (yellow arrow). In both the Active and Passive units the exhaust gas exits at the bottom of the polythene bag (c) and Figure 6. In the Active unit, this pipework runs directly to the fume cupboard for disposal. For the Passive unit, the gas first passes through a second glass tank fitted with a NO2 sensor before exiting to a fume cupboard.

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c..

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Starting conditions

The units did not share services; separate fume cupboards, pumps and gas cylinders were used. Units were planted-up in the same way, and with the same species of plant (three moribund plants were replaced in each unit two days prior to the start of the experiment). Charles Austin1 100 litre/minute air pumps were used to push ambient laboratory air into the Active and Passive unit inlet pipework. The pipework first entered a fume cupboard and NO2 (from a 10 litre Air Liquide gas cylinder with 100 ppm NO2) was bled into the pipework via a T-junction and Tygon tubing; gas then was piped to the inlet mixing chambers. Gas concentrations were detected using Aeroqual 500 data-loggers fitted with NO2 sensors mounted on extension kits. The Active and Passive units had been operated as Aerogation Active Units for over 2 months prior to four days before the start of the experiment, after this date the Passive unit had the water drained out as described above and the pipework dried out by maintaining the airflow.

Sensors were activated just before 9 am GMT to warm-up and prior to assembly of the chambers. The input sensors had ambient air from the input pump passing through their glass enclosures. The exhaust sensors were warmed-up whilst unconnected to the green wall units. The passive exhaust sensor was sealed within its glass tank but with inlet and outlet pipework unconnected to the polythene chamber of the green wall; the Active exhaust sensor was unenclosed and reading ambient air. Immediately prior to the start of the experiment, the air pump on the Passive unit was disconnected from the Aerogation

1 http://www.charlesausten.com/products/aquaticshydrophonics/air-pumps/outdoor/et-series/et100/

Figure 6. The exhaust gas outlets via the base of the polythene chambers for both the Active and Passive units. In the Active unit the exhaust gas sensor is suspended from the last row of planters within the polythene chamber; for the Passive unit it is held in an additional glass chamber (Figure 5c).

pipework. All plants in the Active and Passive units were watered manually, and the chambers assembled as in Figures 4 and 5. The Active sensor was inserted into the polythene chamber and the Passive sensor’s glass enclosure connected to the exhaust pipework. An overview of the assembled chambers is given in Figure 7. At the start of the experiment, NO2 was bled into the inlet pipework at a nominal concentration of 75-80 µg/m3

above the ambient gas concentration indicated by the input sensors. Fine control to ensure that this exact concentration was maintained throughout the experiment was not possible due to changes in ambient conditions and cylinder pressure changes and so some variation in levels was expected.

With the exception of the first day, gas was supplied to the units for 7.5-8 hours during daylight hours (on the first day for approximately 4.5 h). This was partly to simulate the period when the highest on-road NO2 concentrations would be experienced, and partly to have a period of ambient ‘night-time’ air unenhanced with NO2 for comparison. The data-loggers were set-up to record GMT, although during the period of the experiment, the UK was on BST. Sensor readings were refreshed every minute allowing rapid adjustment of the initial gas concentrations, but data was logged at 15 minute intervals. The experiment was started on 22/6/16 and terminated on 26/6/16 – this gave 5 periods with NO2 enhanced air. Sensor readings immediately prior to the introduction of NO2 were used as the baseline to ‘zero’ sensor readings. This was done as individual sensors varied in recorded concentrations and due to inevitable changes in ambient concentrations of NO2 and other contaminants that can react with the sensors, such as ozone; ambient concentrations are likely to vary during the day and night.

RESULTS

Active Aerogation Unit

The five NO2-enhanced sessions for the Aerogation Active system (22-26th June 2016) are given in Figures 8-12. Note that the sensor readings (both input and output) have been set to zero at the reading immediately prior to the opening of the NO2 cylinder and so some negative values can appear. There is typically a high spike of NO2 as the gas level is first introduced and then adjusted to the target range of 75-80 µg/m3. The spike is most pronounced in the first two sessions; in the subsequent ‘sessions’ experience with the regulator sensitivity reduced the spike incidence. The point of cut-off, when the input gas was turned off is very clear in Figures 8 to 11, with a sudden drop in concentration. In Figure

Figure 7. The assembled Active and Passive green walls with inflated polythene chambers. Note the NO2 inlet to the polythene chamber on the Passive unit.

12, there is a tailing-off of the input concentrations, this may be due to reduced pressure in the NO2 cylinder; no evening measurements were taken for the last session (26/6/16). There is typically some creep in NO2 concentrations as environmental conditions over the day vary. To make the graphs more easily comparable, sensor time records have been standardised (in practice they were a minute or two out of synch). The level of NO2 reduction has been estimated from areas of stability following NO2 introduction (indicated by blue shading on the graphs). The mean of NO2 concentrations was calculated (± 1 Standard Error (SE)) for both input and output sensors and these data plus the % difference is given on each of the Figures using inset column graphs. These figures give an estimated reduction of between 57.6% and 68.5% in NO2 concentrations over the five sessions (mean of the five values = 62.4%).

Figure 8. Nitrogen Dioxide (NO2) Concentrations, input and output, from an Active Aerogation Unit (Session 1, 22nd June 2016).

Figure 9. Nitrogen Dioxide (NO2) Concentrations, input and output, from an Active Aerogation Unit (Session 2, 22-23rd June 2016).

Figure 10. Nitrogen Dioxide (NO2) Concentrations, input and output, from an Active Aerogation Unit (Session 3, 23-24th June 2016).

Figure 11. Nitrogen Dioxide (NO2) Concentrations, input and output, from an Active Aerogation Unit (Session 4, 24-25th June 2016).

Figure 12. Nitrogen Dioxide (NO2) Concentrations, input and output, from an Active Aerogation Unit (Session 5, 25-26th June 2016).

Passive Unit

The five NO2-enhanced sessions for the Passive system (22-26th June 2016) are given in Figures 13-17. As with the Active Aerogation units a pronounced spike in NO2 concentration occurs in the first couple of sessions when the gas cylinder is first opened, with input levels stabilising thereafter. The zeroing of concentrations to the sensor readings immediately prior to the NO2-enriched air being entering the system was carried out, as was the standardisation of data-logging time (as for the Active unit above). Immediately prior to the introduction of NO2 into the Passive system in the first session, there is a clear dip in sensor readings for the exhaust sensor, this is probably a result of the exhaust sensor’s chamber being connected to the Passive unit and flushing ‘stale’ air out of the chamber (Figure 13). The point of introduction of NO2 into the system is very clear, as is the point where the NO2 cylinder is turned off.

The level of NO2 reduction has been estimated from areas of stability following NO2 introduction (indicated by blue shading on the graphs). The mean of NO2 concentrations was calculated (± 1 Standard Error (SE)) for both input and output sensors and these data plus the % difference is given on each of the Figures using inset column graphs. These figures give an estimated reduction of between 8.9% and 60.4% (mean of 5 values = 31.2%). However, the initial session (Figure 13) gave an unusually high level of reduction compared with the other sessions which is difficult to account for, and Session 3 (Figure 15) showed a very low level of reduction. If the first session is removed, the mean reduction of the following 4 sessions is 24.0%. Overall, the data showed substantial variability, and there appeared to be a trend for the removal of NO2 to decrease during each session.

Figure 13. Nitrogen Dioxide (NO2) Concentrations, input and output, from a Passive Living Wall Unit (Session 1, 22nd June 2016).

Figure 14. Nitrogen Dioxide (NO2) Concentrations input and output from a Passive Living Wall (Session 2, 22-23rd June 2016).

Figure 15. Nitrogen Dioxide (NO2) Concentrations input and output from Passive Living Wall (Session 3, 23-24th June 2016).

Figure 16. Nitrogen Dioxide (NO2) Concentrations input and output from a Passive Living Wall (Session 4, 24-25th June 2016).

Figure 17. Nitrogen Dioxide (NO2) Concentrations input and output from a Passive Living Wall (Session 5, 25-26th June 2016).

DISCUSSION

The active Aerogation Unit maintained a fairly consistently high ability to reduce nitrogen dioxide concentrations over five successive periods; assuming a nominal input value of 80 µg/m3 this represents a reduction to about 30 µg/m3. To put this into context, the EU annual limit2 is 40 µg/m3 with an hourly limit of 200 µg/m3 which must not be exceeded more than 18 times in a year. The Passive unit, as might be expected was less effective in reducing NO2; at the most optimistic level this might be equivalent to reducing an input of 80 µg/m3 to about 55 µg/m3 but perhaps more realistically to 61 µg/m3. However, the comparison between the Active living wall and the Passive living wall in the laboratory trials reported above may not reflect real-world conditions. An input of 100 litres/minute is within the design parameters for an Aerogation Active living wall system, but blowing that volume across a small Passive living wall is probably unrealistic. Also, the polythene chambers used in the trials were approximately 1 m3 in volume and the vegetation took up quite a small proportion of that (about 0.3 m3; Figure 5c). As a result, there is a very strong likelihood that a substantial amount of the NO2-laden air entering the polythene chamber never came into contact with the vegetation at all, and this likelihood is greater at high pump speeds. The behaviour of the Passive wall system clearly needs evaluating at a range of different air flow rates, and the NO2-laden air needs to be in intimate contact with the vegetation. This latter could be achieved by either having a perforated hose leaching air into the vegetation canopy, by reducing the effective volume of the polythene chamber, or a combination of both approaches (Figure 18). The trend for the exhaust NO2 levels to creep upwards in the Passive unit may be indicative of saturation, and a reduced capacity to remove NO2 as time goes on, but longer trials would be needed to determine whether the trend continues with longer exposure times.

CONCLUSIONS

Both the Active and Passive living walls appeared to reduce NO2 levels from an artificially enhanced airstream. The Aerogation Active living wall removed more NO2 than a similar Passive living wall with the same number, species, and volume of plants. The Aerogation Active wall was more consistent in its NO2- 2 http://ec.europa.eu/environment/air/quality/standards.htm

reducing capability. However, the laboratory test of the Passive system may not reflect real-world operational conditions.

REFERENCES

Dover, J.W. (2016) Pilot Study on the Nitrogen Dioxide (NO2) absorbing capability of the AEROGATION ACTIVE Green Wall System. Unpublished Confidential Report to Treebox Ltd. Green Wall Centre, Staffordshire University, Stoke-on-Trent

Figure 18. Design for passive green wall evaluation with reduced chamber volume and perforated pipe in the canopy to leach NO2-laden air direct to leaf surfaces.