Indicators of System Performance and Design of Units

It should be known that good quality materials or equipment are always recommended for each unit of the system to increase its performance. Indicators of system performance are things or evidence that show or help in showing that a given system are functional as expected.

Total Suspended Solids (TSS), is a measure of the solids that remain in the waste-water after settling has occurred in the tank. BOD and total suspended solids together measure the strength of the waste-water. They can serve as an indicator of system performance.

High TSS can place a great demand on the downstream devices and could lead to clogged components and orifices in distribution manifolds. High TSS can result from:

• The system being under-designed for the source supply.
• Use of low flow fixtures — although they conserve water, they do not reduce the constituent mass loading and result in higher concentrations.
• Use of a garbage disposal.
• Kitchen practices — e.g., kitchen clean-up, food preparation, or cuisine.

Above average use of toilet paper, which can be broken down biologically but only by fungus, which needs air to function. Microbes present in septic tanks typically do not break down paper products which are wood-based.

Laundry machines — due to clothing fibers, clay, or soils present on the clothes. The volume of dirt or grime present in the laundry will directly relate to the habits, hobbies, and occupation of the residents.

Although low TSS is not a problem for the system, it could indicate that something else is wrong with the system. Low TSS could be due to:

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Fewer users on the system than considered in the original design. Higher flows from low TSS sources. Clear water inflows.

The implication of all the above is that in designing the units, they should be made to take care of the TSS and BOD. The expected levels of FOG concentration must be considered during waste-water treatment design. FOG means Fat Oil and Grease‘.

The sides of the baffle should be avoided so that the FOG buildup on the baffle wall is not added into the sample. If a sample is taken from a pump tank be sure to move aside a scum layer if it exists.

Fat

Animal fat is relatively easy to hold in a tank because it is quite sensitive to temperature. It becomes a solid at 80°F, and waste-water temperature is usually less than 80°F. Animal fat will break down in the soil, but it takes four times more energy to break down than the organic matter typically measured by BOD5.

Fat is added to the system from cooking, clean up, and dish washing, so commercial systems will typically have higher levels of fat than residential systems. If a system is supplied with a lot of animal fat, it will typically stay in the septic tank.

If it is contained in the septic tank, it may not be observed in FOG measurements in downstream components.

Oils

Vegetable oil is not as sensitive to temperature as fat and can pass through the system. Oil can also be broken down through a biological process, but it takes 12 times more energy to break down oil than the organic matter typically measured by BOD5. There are many different types of oils used, but vegetable is the most common.

Vegetable oil is often used in the liquid form, but it can also be solid shortening. The liquid form is harder to hold in a tank. The ability of the oil to separate is influenced not only by temperature, but also by how the oil was generated and used.

Free oil will rise to the waste-water surface and be easily separated when the mixture is allowed to become quiescent. Emulsified oil has been broken up into very small droplets and occurs either by mechanical or chemical action.

An example of mechanical emulsification is when extremely hot water from a dishwasher is mixed with the oil. Given time and a decrease in temperature, this oil can be separated.

Chemical emulsification occurs when detergents or cleaners produce a mix of oil and water. Degreasing compounds can generate dissolved oils, in which discrete oil particles are no longer present.

Chemically emulsified oil will take a longer time to separate, increasing the risk of carrying it to downstream components unless long quiescent periods are available to allow separation.

Grease

Grease is petroleum-based and can be toxic to a system. Because grease is petroleum- based, it cannot be broken down, but it can be separated. Grease comes from lotions, hair products, and soaps.

Typically, there will be a higher percentage of grease in the FOG from residential systems when compared to most commercial systems. Grease can build up over time, coating components and inhibiting treatment of other constituents in the waste-water.

Temperature

Septic tank effluent on average is approximately 20 degrees (°F) warmer than the ambient ground temperature. Microbial activity doubles in population every time the temperature increases by 18°F (10°C) until the optimum temperature is reached. As the microbial activity doubles, the biodegradation of constituents increases.

This means that oxygen uptake is more rapid at warmer temperatures, requiring air to be supplied at a higher rate. The waste degrades more quickly at warmer temperatures, so it need not be held in the treatment system as long when it is warm.

The converse is also true: in the winter, oxygen uptake is low and air need not be supplied as fast.

However, the waste takes longer to degrade, and would thus need to stay in the treatment system longer during cold months. The practical implication of this is that aerators are designed using summer temperatures and detention tanks are designed using winter temperatures.

Forecasting Average Flow Rates

The development and forecasting of average daily flow rates is necessary to determine the design capacity as well as the hydraulic requirements of the treatment system.

The maximum daily flow rate is important particularly in the design of facilities involving retention time such as equalization basins. Data on peak hourly flows are needed for the design of collection and interceptor sewers, waste-water – pumping stations, waste-water flow meters, sedimentation tanks and channels in the treatment plants

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