A more sustainable design is to install closed loop irrigation system. Here, the water that passes down the plant wall is collected in a reservoir to be reused. Being a closed system, it also means that the nutrients in the water that were not taken up on their pass through the root zone will be available to the plants on their next circuit rather than being ejected into the environment via the drain. Rather than relying on the pressure from the domestic water system to deliver the water to the top of the wall, a closed loop system uses a pumping system to lift the water to the top of the wall. The reservoir may be integrated in the base of the wall or may be remote.

There were fears that the cycling of water repeatedly through the growth media would encourage the proliferation of root diseases. As in agricultural hydroponics, it is now believed that this fear is overrated. Root diseases are problematic when the plants are already under stress such as when the plants are being flooded (overwatered). Because of transplant shock, newly established plant walls are susceptible to root diseases and care should be taken at that time. With good plant care, root diseases offer little threat.

A certain volume of water is lost to the vapor state as it circulates through the system largely via transpiration of the plants. Therefore the reservoir needs to be topped off either manually or automatically with either a mechanical or electronic system to replace the evaporated water. Alarms can be installed to warn when the levels in the reservoir are either excessively high or excessively low. Systems can also give warning when there is a pump failure to ensure the wall does not dry. To minimize the risk associated with mechanical failure of the circulating pump, it is frequently advantageous to install a second pump and, similar to many other mechanical installations, have them arranged in a lead lag configuration. Integration of the irrigation system into the building automation system ensures good protection of the owner's investment into the plant wall.

Construction of a Hydroponic Indoor Air Biofilter

The construction has three major components: the basin, the infrastructure, and the plants.

Although the basin probably has the greatest range of design choices, its construction is straightforward. The basin is frequently a poured concrete or metal container, and may be recessed below or on the slab. The exposed surface is commonly water-proofed with a trowel-on, two-part liquid membrane. The basin has two functions—first, as a catchment for the water flowing through the biofilter and second, as a reservoir for this circulating water. Although the two functions can be physically separated (e.g. the base of the wall functions as a catchment which directs the water to a reservoir located potentially some distance from the biofilter), the basin is more typically both the catchment and the reservoir. Pumps located with the basin/reservoir, lift the circulating water from the basin to an emitter system that disperses the water across the top of the wall at a rate of approximately 4 quarts per minute per yard (4 liters per minute per meter) of width of the wall.

The infrastructure component of the living wall biofilter can be divided into the air diffuser and the hydroponic growth media. The function of the diffuser is to ensure uniform air flow through the porous hydroponic media. The diffuser is an array of vertical perforated ducts which are connected to the return duct of the building via a horizontal manifold. The perforated ductwork is installed as overlapping corrugations on metal panels which are shingled into the wall.

Installing the infrastructure of an indoor biofilter system and connecting it to the HVAC system of the building. Photo courtesy of Nedlaw Living Walls, Inc.

The diffuser of most multifloor systems has manifolds connecting to the HVAC system on each floor. This is done to reduce the size of the perforated ducts (minimizes the footprint of the system) and allow the different air flow rates through the biofilter on each floor (for example preferentially drawing air from the top of an atrium during the cooling season). Alternatively, built-in fans can draw the room air through the root zone and expel the air back into the occupied space. This approach is somewhat less efficient than being directly tied into the HVAC but may be the method of choice with retrofits or smaller systems.

Since the semi-rigid growth media offers very little resistance to air flow because of its open mesh design, the sizing of the manifolds, internal ductwork, and perforations in the ducts must be carefully engineered to ensure even air flow through the biofilter with minimum footprint from the floor plate for a given air flux through the filter. For larger systems, this must be done in consultation with the mechanical engineers for the building. The growth media which comes in rolls is physically fastened directly to the diffusers with stainless steel fasteners.

Installation of the growing media in two layers over the infrastructure of the biofilter. Photo courtesy of Nedlaw Living Walls, Inc.