Canadian Consulting Engineer

Feature

indoor Air Quality: Living Walls: Art and Science

It is generally understood that greening a building’s indoor space can have a favourable impact on the wellbeing of its occupants. The possible benefits read like a business manager’s wish list: greater...


It is generally understood that greening a building’s indoor space can have a favourable impact on the wellbeing of its occupants. The possible benefits read like a business manager’s wish list: greater productivity, reduced absenteeism and lower overall stress levels. Add to that the latest research confirming that people seek out natural settings, and the increased use of biophilic designs is no surprise at all.

A living wall biofiltration system is a vertical hydroponic plant arrangement that relies on a combination of mechanical and natural processes to help clean the air within buildings and create beautiful, comfortable spaces for building occupants.

While a living wall biofiltration system may not garner specific LEED points, it can help fulfil a building’s LEED potential by improving indoor air quality and enhancing indoor spaces.

Why use an indoor air biofilter? Most non-residential buildings have fixed windows and HVAC systems designed to “ventilate” a building. Outdoor air is drawn in, heated or cooled to match indoor conditions, then filtered and distributed to the occupied spaces. Using mechanical filters, the process dilutes and, over time, removes a portion of the airborne pollutants. However, it typically cannot remove VOCs, gases and odours.

Biological systems can clean air of most types of pollutants. And unlike mechanical systems, they don’t need parts to be changed every six months. Also no energy is required to condition the return air beyond what is normally required.

Although biofiltration has been tested as a means of maintaining air quality in a variety of spaces, including residential and agricultural buildings, to date most installations are found in commercial or institutional spaces.

Do they work? Results of a study

Biofilter performance was the subject of a recent investigation undertaken by the Controlled Environment System Research Facility in the School of Environmental Science at the University of Guelph. The first part of the study was a randomized field trial. Here, two portable biofilters were cycled between four buildings (two single detached houses and two small office buildings) for four cycles of roughly 10 days each. At the end of the experiment, each building had two sampling periods with the biofilter and two without.

Using 3M Organic Vapour Monitors (OVM), the concentration of tVOCs (total volatile organic compounds) in the spaces was measured with and without the biofilter and exhaust air stream from the biofilter.

The presence of a biofilter in the space lowered tVOCs from 204 to 113 µg/m3. The biofilters were extremely effective at removing alcohols such as ethanol and iso-propyl alcohol. They were also constituently able to remove aromatic compounds such as BTEX, styrene and cymene. Terpenoids, limonene and pinene were also effectively removed. Surprisingly, the long chain alkanes, as well as decane, octane, heptane and hexane were also degraded. The biofilter had little impact on chlorinated compounds, however.

A follow up experiment evaluated the performance of 12 biofilter installations. These biofilters were actual commercial installations ranging in size from less than 1 m2 to over 150 m2, and ranging in age from a few months to over seven years.

A portable PID VOC sensor was used to measure the reduction of tVOC concentrations as the air stream passed through the biofilter, as well as the amount of air treated. In this study, a single pass through the biofilter reduced tVOCs by approximately 85%, effectively lowering them to outdoor concentrations.

By also measuring the air flux through the biofilter, it was possible to calculate the biofilters’ Clean Air Delivery Rate (CADR) – that is, the product of the amount of air treated and its quality. The biofilters were found to have an average CADR of 40 litres of virtual outside air per square metre of biofilter.

The delivery rate was found to be limited almost entirely by the size of the mechanical system drawing the air, not the system’s biology. With larger fans, the biofilters could easily deliver between 80 and 100 litres of virtual outside air per square metre of biofilter.

In this way, virtual outside air can be added to the make-up air calculation for the space, allowing the occupants to enjoy a higher IAQ without increasing the amount of outside air being brought in. This improvement in the indoor environment can be recognized as roughly $2,000 per m2 of biofilter (depending on the size of the system). The results of this study are currently being prepared for a peer reviewed publication.

Constructing a living wall

A living wall is constructed using three major components: the basin, the infrastructure, and the plants.

The basin functions as both a catchment for the water circulating in the biofilter and as a reservoir for this water. Submersible pumps located in the reservoir lift the circulating water to an emitter system that disperses the water across the top of the wall at a rate of approximately four litres per second per metre of wall width.

The infrastructure component of the biofilter includes the air diffuser and the growth media. The diffuser is an array of vertical perforated ducts which are fastened directly to the support wall (typically block) using concrete anchors. The diffuser ensures that uniform air flow is drawn into the porous growth media. Typically, the diffusers connect to the return air of the HVAC on each floor, allowing differential air flows through the different heights of the biofilter.

Air flow and systems

Since the semi-rigid growth media offers very little resistance to air flow, it is important that the size of the manifolds, internal ductwork, and perforations in the ducts are carefully engineered to ensure there is an even air flow into the biofilter. The systems are typically designed to flow between 0.05 and 0.20 m3 of air per m2 of biofilter per second with a pressure of less than 0.5” across the system. The flow rates are determined to supply enough “virtual outside air” to replace or augment a significant component of the normal make-up air for the space.

Moving the potential volumes of air through the biofilter means the system has to be fully integrated into the building’s mechanical design, while control and monitoring of the biofilter must be interfaced with the building management system.

As the return air comes in contact with the rooting substrate, contaminants move into the water phase where they are broken down by the beneficial microbes. After the air is actively drawn through the plant wall by the HVAC system – conditioned in terms of its temperature and humidity – it is returned to the occupied space.

Installation and plants

The growth media is mounted directly to the internal diffuser system with stainless steel fasteners; it goes on as two staggered layers, each about 2 cm thick. Since the media has very little nutrient holding capacity, nutrients for the plants and microbes must be delivered via the circulating water in the form of hydroponic fertilizers. Water from the pumps trickles down between the layers, creating a vertical hydroponic system.

Although the infrastructure is typically installed at the final stages of the rough construction (when the drywall has been taped), planting usually happens closer to the occupancy date. Rather than pre-growing the plants in the growth media, mature potted plants are obtained from the open market. The plants are carefully bare-rooted to remove the soil from the root mass and then transplanted into the biofilter. After little more than a few weeks, the transplanted plants have re-established their root systems to the point where they cannot be easily removed.

The plants used in the biofilter fall under the general category of “foliage” plants which include Ficus spp., Dra
caena spp., Philodendron spp. and Syngonium podophyllum. As each type of plant has a number of species and/or varieties, living walls actually use more than 30 different types of plants.

The plants for the growth media are selected based on four criteria: their ability to form good relationships with the beneficial microbes that actually clean the indoor air; their ability to tolerate the unique conditions of the vertical system; how closely they match the light, temperature and water conditions of each installation; and design. Leaves of varying colour, shape and texture are used to give a distinctive look.

Integrating with the architecture

A living wall is most effective when the biofilter is integrated into the space. This requires careful coordination with the entire design team, starting with simple sizing of the biofilter for the space. A starting point is 1 m2 of biofilter to 100 m2 of the floor space to be treated.

Lighting is also a key factor for the success of the system, and supplying enough natural and artificial lighting for plant growth has both architectural and electrical considerations.

Maintenance, pest control

and feeding

As with any mechanical component, the living wall biofilter requires routine maintenance. The systems are typically serviced monthly. Seventy per cent of this service is similar to what is required for any interior plantscape — pruning, fertilizing, pest control and plant turnover.

Maintenance draws on standard agricultural practices, with particularly heavy reliance on organic solutions, such as the biological control of pests (“good bugs” eating the “bad” ones), rather than petrochemical solutions. However, because of odour concerns, organic nutrients are not routinely added. The remaining 30% of the maintenance is related to the hydroponic nature of the system — checking flow, controllers and pumps. The tasks can be divided between the landscape and building facilities personnel.

A breath of fresh air

The most successful buildings make people feel more comfortable. These happier people become more productive and, in the long run, help their companies become more profitable. One challenge of building design is to meet and solve that calculation. Adding living wall biofilters into the mix can make the task easier. Alongside the glass, concrete and steel that fill our cities, living walls offer an indoor breath of fresh air, naturally. cce

SIDEBAR

University of Windsor Centre for Engineering Innovation

The photo opposite shows the three-storey living wall at the University of Windsor’s new Centre for Engineering Innovation designed by B&H Architects.

Comprising nearly 110 square meters of living plants with an artistic appearance of flowing water, this Nedlaw living wall biofilter is located in the building’s main atria and serves as a backdrop to the main staircase.

The biofilter is integrated into the building’s HVAC system and includes a series of perforated internal diffusers with a maximum flow rate through the living wall of 0.07 m/s. The wall functions as a vertical hydroponic garden. Water trickles evenly down the interior of the wall at a rate of 40 litres per minute, is collected in a basin below and then is circulated back to the top by two pumps.

The water levels in the basin are monitored by the BAS, ensuring sufficient hydration is available for plant survival. The estimated water demanded by the biofilter is about 0.2% of the building’s plumbing fixture water use.

For B&H principal Kevin Stelzer, biofilter walls are a sustainable technology with enormous potential. “They give us the combined effect of substantially higher indoor air quality while providing a delightful architectural amenity,” he says.

Halsall provided green building and structural engineering services.

Alan Darlington, Ph.D., is vice president of Nedlaw Living Walls, near Waterloo, Ontario. His companies have installed over 150 biofilters across North America and he holds patents related to the technology. Emma Rohmann, P.Eng., is a project and team manager in the green building and energy services team at Halsall in Toronto.