In recent workshops, we’ve been asked if flow equalization is a legitimate concept for managing wastewater in residential situations. So we decided to take the time to introduce the concept and some of its applications. As system management becomes more common, installers will be involved with systems employing this design concept.

Flow equalization is basically storing wastewater generated over a period of time to spread the flow out more evenly for a day or week. It is the start of managing treatment efficiency in the system. The desired flow rate – or daily loading – is determined by the use pattern and levels, and the ability of the final soil treatment unit to accept the wastewater.

Management can be approached from two different perspectives: working with users to change their habits and reduce overall flow, and management of flows within the system to improve efficiencies. If designers find themselves faced with these situations, a combination of the two approaches is chosen. Water-saving fixtures will be installed to reduce overall flow and, in particular, to lower peak flows.

One point to remember is that just reducing the overall flow or peak will not typically change the overall organic loading or the level of nutrients or pathogens. So it only addresses one part of our big balancing act between accepting the wastewater to the system and having treatment take place.

GO WITH THE FLOW

In addition to managing wastewater flows by changing water-use habits, peak flows can be managed through flow equalization tanks. Although this method does not change the amount of flow from the facility, it does even out the flow into the treatment system. There are many situations where flow equalization can help system operations.

It is probably a good idea if the average flow is consistently at or above 70 percent of the design capacity. If there is great variability in water-use habits or the hours of operation in the facility are variable, flow equalization may be a good choice to bring more consistency to the flow. An example of this might be a church being used only a few hours a week. If the peak flows are frequently significantly greater than the average or design flow, then flow equalization is probably necessary. Other situations where this may occur are with restaurants and bars, and even in some homes where there are a lot of visitors or entertaining.

We are always reminded of a trip we made to Malibu, Calif., where we highlighted in a workshop that although there may be significantly more flow in a domestic residence in the morning before work and school and then again in the evening, most systems are set up to handle these “normal” variations. Seminar attendees told us that when their clients throw a party, they throw a party! This means they need to deal with very high peak flows.

Of course, even in a residence where the flow varies from day to day – due to wash day or use of cleaning services – and the system is being loaded on average more than 70 percent of the design flow, equalizing the flow over 24 hours will improve system operation. Using timers to regulate distribution of sewage through the day also means that the system cannot be overloaded for a significant period without setting off an alarm.

UNCOVERING PROBLEMS

A flow equalization tank does more than average out water delivered to the treatment system. A well-monitored system using timed delivery can help uncover problems in the system. Major changes in flow patterns can detect leaking tanks or devices using too much water, or clogged orifices in the distribution system. A system with a timed surge tank designed to handle 350 gpd may activate a high-water alarm if discharges are consistently more than that design flow.

Several factors can contribute to excessive flows, including undersized components, leaky fixtures and/or groundwater/surface water infiltration. Or in the case of clogged orifices, the pump will be prevented from delivering its designed output. For example, a pump may be designed to deliver 30 gpm. However, as the orifices begin to clog, the actual volume delivered by the pump will decrease. Since the timer is set based on the pump delivering 30 gpm, the pump will not keep up with the system output of 350 gpd and the high-water alarm will activate.

In the event of an extended power outage, the tank will provide storage and prevent all the collected water from being immediately sent to the treatment system upon startup.

Lastly, if some of the more advanced pretreatment components such as media filters or aerobic treatment units are a part of the system, microbes in the system will be much happier having food delivered to them on a regular basis. This will improve treatment by these components, which should benefit the soil treatment unit.