There’s a lot that goes into loading rates, and the more you understand them, the better service and education you can provide customers.
The amount of wastewater entering the treatment system is the hydraulic loading rate. When sizing for the hydraulic loading rate, the volume of water flowing through the treatment process is the design parameter under consideration.
Many state rules and design guidance documents only refer to the hydraulic loading of onsite wastewater treatment systems, but the amount of contaminant (organics, solids, nutrients) load in the wastewater is a very important and often left out factor.
Understanding Concentrations
In general, raw residential wastewater should have a biochemical oxygen demand of less than 300 mg/L, total suspended solids of less than 200 mg/L, and fats, oils and grease less than 50 mg/L. In residential applications total nitrogen is generally less than 60 mg/L, and total phosphorus less than 15 mg/L.
But that’s not always the case, residences with low-flow fixtures, commercial properties with toilet flushing as its primary source, and buildings with above normal food preparation will likely have higher values. High BOD concentrations can be caused by high organic loading to the system.
In a residential system, the number of people in the house could be greater than what the system was designed for and originally constructed. An elevated BOD concentration could also be influenced by the activities that are happening at the source. In a home, a large portion of BOD is generated from the toilet. Toilet waste also contributes a large part of the natural microorganisms, nitrogen and phosphorus. In some homes or restaurants, the presence of a garbage disposal, the types of foods prepared and methods to prepare them can increase the BOD concentrations.
Elevated TSS values can also be related to excessive food waste and toilet waste, but may also be connected to laundry. TSS measures both the organic and inorganic solids, and the dirt off our clothing along with the fibers that are shed during laundering can both contribute. Another cause is excessive toilet paper usage as toilet paper is designed to break down under aerobic conditions which are not present in the septic tank.
High FOG in domestic wastewater generally originates in the kitchen from disposing of fats and oils down the sink drain. The largest source of nitrogen and phosphorus in wastewater is directly from human waste and food waste, so generally elevated levels are seen in facilities that dispose of food waste and those that have a high percentage of the wastewater generated in the bathroom from frequent toilet flushing.
Mass Loading
The mass loading is a calculated value resulting in the number of pounds of the containment per day. A designer can calculate the estimated or actual organic loading using BOD, the solids loading rate using TSS, or the nutrient loading using the total nitrogen or phosphorus.
To perform this calculation the professional needs to either estimate or measure the daily generation of wastewater (gpd) and the concentration. Whenever possible, collecting actual water use data and wastewater samples is preferable. In addition, many state codes build a safety factor into their flow estimates, with many residential and commercial systems utilizing significantly less flow than estimated. Being able to characterize the source assists in identifying the expected quantity and quality of the wastewater to be treated. The actual organic loading from a source is heavily dependent on activities within the source as well as management practices.
Hydraulic and the mass loading rates are related, and both must be considered when designing and evaluating an existing system. The mass loading rate is just as significant to the design and operation of an onsite wastewater treatment system. If there are high organics, solids or nutrients loading to the system, this could drastically change the sizing of treatment components for a given flow.
Use of low-flush or water-saving devices is encouraged in homes and commercial properties to reduce the amount of water that is being used for daily activities. If there is a reduction in hydraulic loading to the onsite wastewater treatment system, there is no reduction in the amount of contaminants going to the wastewater system. The concentration of the constituent in the wastewater increases when the hydraulic flow is reduced. On the other hand, reducing the flow results in an increased detention time; an increased detention time can improve the treatment capacity of certain components.
Calculating Mass Loading Rate
To calculate the mass loading you take the concentration and multiple by the flow including a conversion factor to balance out the units:
Pounds of contaminant = concentration (mg/L) x flow (gpd) x 8.35 ÷ 1,000,000
For example, if a convenience store was using 400 gpd of wastewater and the measured BOD was 500:
400 gpd x 500 mg/L x 8.35 ÷ 1,000,000 = 1.7 pounds/day of BOD
A treatment system then needs to be designed to not only manage the hydraulic load of 400 gpd, but also the elevated organic loading, which is double what you could expect from residential application. If either of these values increase the mass of organics will go up. This same calculation can be done with TSS or nutrients.
The last issue to address is the constituent mass loading with respect to water quality. The most common concentration standard is for nitrate nitrogen. The federal drink water standard identifies 10 mg/L nitrate as the limit to prevent problems during pregnancy as well as methemoglobinemia or “blue baby syndrome,” a disease in infants that reduces the blood’s ability to carry oxygen.
Since most sampling of onsite wastewater treatment systems are “grab” samples we have limited data on the concentration of the effluent over time. Assessing the load of a nutrient discharged may be a more accurate assessment of a decentralized systems’ contribution to water quality than measuring the concentration of these pollutants. Mass considers not only the amount of a substance present, but also the total volume, giving a more accurate picture of the overall quantity of a substance. This is especially true when dealing with environmental impacts where the total mass of a pollutant is more relevant than its concentration in a specific volume.
For example, let’s say we have a system with a nitrogen limit of 15 mg/L that has a peak design flow of 4,000 gpd. The mass loading of nitrogen from this system is 0.5 pounds of nitrogen per day. In reality, when the facility only uses 1,800 gpd averaged out using flow equalization and the concentration of nitrogen out of the system is 20 mg/L the mass loading of nitrogen is 0.3 pounds of nitrogen per day which is well below the per limit on a mass loading basis.
As designers, engineers, soil scientists, regulators, manufacturers and installers we need to think about much more than the hydraulic loading specified in regulations and the mass loading to our systems and to the environment.
