Green Roofs - Living Architecture Monitor - Winter 2011 - (Page 26)

RESEARCH FIGURE 2 Impact of bioﬁltration on IAQ. The graph presents two lines. The ﬁrst line indicates concentration of VOCs in a space under a range of ventilation (typical ventilation). The concentrations are presented relative to those seen when the space is subjected to ventilation rates of 7.5 litres per person per second. The second line is the relative concentration when the space is subjected to the same range of ventilation but the air in the space is also treated with a standard-sized bioﬁlter (ventilation + bioﬁlter). RELATIVE VOC CONCENTRATION 250% TYPICAL VENTILATION VENTILATION + BIOFILTER 200% ever, biofilters in this study with higher rates of flow air (for example 0.1 m3air m2biofilter s-1) had CADRs between 80 and 100 litres per second. The study clearly indicates that the CADR was limited by the mechanical components capacity to draw air through the biofilters. Typically the biofilter removal is a “contact time” phenomenon; the longer the contaminant is in contact with the biofilter, the more likely it is to be degraded. The faster the airflow, the lower the removal efficiency. This decrease in removal-efficiency with increased air flux was not seen with observed airflows during the study. IAQ IMPACT In what ways do biofilters impact IAQ? The final IAQ in a space is largely a factor of how fast the VOCs are generated in the space and the speed at which they are removed. The rate of VOC generation from materials and activities in a space is fairly constant over the short-term. On the other hand, VOC removal in most buildings is conducted via ventilation. (The impact of changing the ventilation rate with a constant VOC generation rate on IAQ is presented in Figure 2, at left) VOC concentration observed with ventilation rates of 7.5 litres per person per second are used as a reference point. When the ventilation rate is increased within a space with a constant VOC generation rate, the equilibrium concentration of VOCs in the space will be lowered. To determine the impact of a biofilter in a building it must be properly sized. Although there are a number of different ways of sizing biofilters, a quick estimate is to use five occupants per square metre of biofilter. To calculate CADR, a standard flux rate through the biofilter is 0.05 m3air per m2biofilter per second and a removal efficiency of 60 percent is assumed (a conservative estimate based upon the above study from the University of Guelph). The impact of this CADR on IAQ over the range of ventilation rates can be determined using the models of Guieysse et al. (2008). The addition of the biofilter to the space greatly improves the IAQ in terms of VOC concentrations for all ventilation rates. Data in Figure 2 further demonstrates two modes of operation by which the biofilter can function within the building: 1. AUGMENT The first mode of operation is 150% 100% 50% 0% 0% 5 10 15 20 VENTILATION RATE (LITERS/S/PERSON) Source: Alan Darlington, Nedlaw Living Walls from a few months to over six years-old; and with air fluxes between 0.01 to 0.1 m3air per m2biofilter per second. With these living walls, researchers then evaluated the percentage of VOC contaminants removed per pass through each of the 12 biofilters by: (1) comparing the total concentration of the different VOCs present in the air entering and exiting the biofilters; (2) and rate of air flow. The study yielded impressive results: although the concentrations of VOCs in the indoor air varied considerably between sites, drawing the indoor air through the biofilters removed an average of 85 percent of the VOCs present. In fact, the biofilters were able to clean the air to the point at which VOC concentrations were not significantly different for outside air; VOC concentrations of the exhaust from the biofilters was 16 ppb ± 24 versus outside values which were 11 ppb ± 18. Results also showed there was no correlation between the age of the biofiltration system and its performance; systems only a few-months-old 26 LIVING ARCHITECTURE MONITOR WINTER 2011 functioned as well as those six-years-old. Researchers further found that the percentage of VOC material removed per pass was not dependent on the size of the system. The amount of air treated by the system was dependent on its size, but the quality of air generated is not dependent on the size of the actual biofilter. The rate of clean air delivery was also measured by researchers. Generally speaking, the impact on IAQ is a function of both the quality and quantity of air treated: the clean air delivery rate (CADR) is a term used by the Association of Home Appliance Manufacturers to describe this. CADR, then, is the product of the amount of air treated by the filter and how well the filter cleans the air. For example, a filter that treats one-cubic-metre of air per hour and removes 10 percent of the contaminants has the same impact on air quality as a filter that treats half-acubic-metre of air per hour but removes 20 percent of the contaminants. In the Guelph study, the biofilters had an average CADR of approximately 40 litres of air per square metre of biofilter per second. How- to keep ventilation rate the same and use the biofiltered air to augment what is being brought in from outside. Under this scenario, the occupants will enjoy a substantially higher IAQ with nominal increase in energy consumption. For example, the addition of a biofilter to the HVAC

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Winter 2011

Green Roofs - Living Architecture Monitor - Winter 2011
Contents
The Living Architecture & Health Connection
Picnic Perfect
Green Roof Sight-Seeing
A Sacred Space
On the Roof With… Judith H. Heerwagen
A Spiritual Oasis
Therapeutic Landscapes
Active Living Walls
Lifetime Achievement Award: A Legend Remembered
Civic Award: Kelly Luckett
Research Award: Jeremy Lundholm
Fieldnotes From Greenbuild 2010
Toward Net-Zero Water Use
Learn Online
New Corporate Members
Welcome New GRPs
Cents and Sustainability

Green Roofs - Living Architecture Monitor - Winter 2011