On-site Wastewater Treatment Technology – Part 1
Recently I received a comment from a reader who was looking for information on on-site wastewater treatment systems available in Canada. The requirements were fairly specific; scalable from 4 to 40+ people, and able to treat domestic wastewater to re-use quality. Each criteria alone would have been easy to meet, but together they presented an interesting challenge. The solution I found is very promising, and gives me many ideas for incorporating high efficiency wastewater treatment into my future projects.
On-site wastewater systems have been around for as long as people have had places to call home. Originally, they consisted of little more than a hole in the ground, and while they look different now, functionally speaking some types haven’t really progressed much beyond that. We’ll be looking at 2 major types of systems in this post; septic systems and other ‘advanced’ wastewater treatment systems.
Anyone with a cottage or other rural property will be familiar with septic systems, and many will also be familiar with not only their limitations but also the problems that can arise when they malfunction. Depending on the number of users and some characteristics of the available land, a septic system can definitely work. Assuming a normal family size of 2-4 people, good drainage properties of your soil, and sufficient distance from drinking wells, bodies of water, or other environmentally sensitive areas, a drain field will fit on an average sized rural lot. This space does have other limitations though; it cannot be used for a garden, nor can a driveway be built on top of it. If space is at a premium, then these restrictions may require an expensive engineered solution, if it’s even possible. The organic matter in the wastewater is broken down in a settling tank, but the majority of the treatment occurs in the soil beneath the drain field. This means there is contaminated water entering your lawn, which is hopefully cleaned by the time it reaches the water table. By default, this removes any possiblity of water re-use, something that the reader indicated was essential.
System capacity isn’t very scalable either; more people requires not only a larger septic tank, but also a larger drain field. Because one of the requirements also presented was systems that could handle 40+ people, a septic system didn’t seem like a viable option for this reason either.
Next, I looked at advanced treatment systems, with a specific focus on membrane bioreactors (MBRs). This is where things started getting interesting. MBR technology is actually a hybrid system, using both biological treatment and mechanical microfiltration to clean wastewater. Like a septic system, MBRs use a large tank to collect wastewater, but this is where the similarities end. Septic systems rely on anaerobic (without oxygen) bacteria to break down biological matter, a process that can produce very unpleasant smells. In contrast, aerobic wastewater treatment has very little smell. I’ve personally stood very near the aeration tanks of several sewage treatment plants, ranging from community scale down to a single family system, and while I may not want to have a picnic there, there was no difficulty being there for fairly extended periods of time. Because the aeration breaks up the biological matter, the aerobic bacteria are able to eat it, leaving very little solid waste. This means MBRs don’t need to be pumped out on a regular basis like septic systems do, which is a big cost savings.
Membrane bioreactors offer 2 major benefits over septic systems; since they don’t require large drain fields for biological treatment, scaling the systems is as simple as increasing the size or number of aeration tanks. They also produce very clean effluent, which is immediately available for re-use. Similar to common household Reverse-Osmosis (RO) water filtration systems, the membranes are composed of a thin porous material, with pore sizes between 0.1 and 0.4 microns. 1 micron, or μm, is one millionth of a metre. To put that in perspective, bacteria sizes range from 1-10 microns, and a single strand of spider web silk is about 8 microns thick. By having a pore size of less than 0.4 microns, nothing passes through the membrane except clean water and a few viruses (which are easily destroyed by an ultraviolet sterilizer on the outfall of the system). The resulting water is clean enough to be used immediately for domestic service water applications, such as irrigation, toilet flushing, sprinklers, car washing, and even washing machines. This has the direct benefit of reducing freshwater consumption by up to 50%; a huge savings if your water supply comes from a well or is metered.
In On-site Wastewater Treatment Technology Part 2, I’ll discuss the 3 MBR systems that I researched, and outline why I recommended one of them over the others.