Tech Talk

An In-Depth Look at Onsite Wastewater Treatment Systems

“As we approach the third decade of the 21st century, we face new challenges to protect our limited resources for future generations.”

We push to design more sustainable buildings, with goals such as achieving net-zero energy, building with recycled materials, and reducing building waste. As a result of the effort to introduce novel sustainability technologies, one particular resource has been gaining increased attention in the public eye: water.

News headlines continue to remind us of water shortages spurred by more frequent and longer droughts. These ever-present warnings have been a catalyst for building codes, owners, and designers to begin pushing for water efficient buildings. While new technologies and strategies for water use reduction have started to make their way to market, how do we find the best options to employ in buildings? What are some of the advantages and drawbacks of various water-saving technologies? In this article, we will dive into the pros and cons of one water-saving strategy: onsite water treatment and reuse.


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Affiliated Engineers, Inc.

Rising Costs of Water in the United States

On-site treatment can be specific to each type of building wastewater - reclaimed rainwater, greywater, blackwater, or a combination of all three. Rainwater is precipitation on a controlled area, free of major pollutants or contaminates, and includes cooling coil condensate. Greywater is relatively clean wastewater from equipment and restroom fixtures, that is free of human waste. Finally, blackwater is collected from sources containing human waste such as a toilet or urinal. All these waste types can be collected and treated within the producing building for reuse in mechanical equipment makeup, irrigation, toilet and urinal flushing, and other uses that do not require potable water. Each of these waste streams presents advantages and challenges when reused within a building.

Rainwater requires the least treatment of the three sources and is, therefore, a relatively simple system. Challenges do arise when rainfall seasons are infrequent or misaligned with reuse needs. In many areas of California, rainfall is limited to around a dozen inches a year and often occurs in the winter. This misalignment causes a building intending to collect rainfall for irrigation to need a large tank to catch and store winter rains for the summer. These large tanks have a high price tag and take up a large amount of space. However, rainwater reuse may be more suited for a building in Florida, as the state has higher rainfall rates throughout the year.

Although greywater requires more treatment than rainwater, it is a more reliable source of water. Building occupants can be expected to use handwashing sinks or other possible collection sources throughout a building’s operating hours. Treated greywater can then be quickly reused for processes such as toilet flushing. The primary issue with greywater reuse is balancing the greywater production with the final use’s water needs. Therefore, a gym collecting a lot of wastewater from showers will find more benefits from greywater reuse than an office building only collecting small amounts of waste from restroom sinks.

In contrast to greywater, blackwater reuse allows for the collection of all wastewater generated in the building. This source produces the most reliable and largest quantity of wastewater; however, it requires intensive treatment to be safe for reuse. Blackwater is understandably the strictest waste treatment system type requiring continuous monitoring to maintain water quality. Treatment requirements often lead to a high price tag, making them difficult to fit into a construction budget.

Onsite wastewater reuse is becoming a common practice in the United States and globally. Cities such as San Francisco are requiring new buildings over 250,000 sq ft to review suitable water reclamation options and install treatment systems. While these systems have the potential to save large quantities of water through a building’s life, they require other limited resources to function, such as electricity, materials, and land.

Water Use and Sources