By Claude Goguen, P.E., LEEP AP
Editor’s Note: This is the first article in a three-part series on septic tanks.
Being a producer of precast concrete septic tanks can be tough. Depending on where you are located, competition from alternative products or other precast producers can be fierce. How do you set yourself apart? You know your plant makes a great tank and focuses on quality control. You’ve got a wonderful new website and can’t lower your price much more. Where do you go from here?
One option is to expand your expertise and services to offer more than just a tank. You can go from asking, “What size do you need?” to asking, “What are you treating?” Some customers look for producers who can help with treating high-strength wastewater, some want guidance treating a sensitive area that requires nitrogen reduction strategies and others may be concerned about pathogens. Evolving from a product supplier to a solution provider can help you rise above your competition. Here’s some information on what’s ahead should you choose to venture down this path.
It starts with knowledge about the system
In the scheme of wastewater treatment, the septic tank sometimes gets shortchanged. It’s viewed as simply a buried box that holds sewage, but it’s much more than that. It’s the first stop in a wastewater treatment plan. This tank houses a complex system of microorganisms and bacteria to create a mini ecological system that lowers the wastewater strength prior to proceeding to the system’s next phase. Learning about these processes goes a long way in making you a resource for homeowners, business owners, contractors and regulators.
What is in wastewater?
When you consider that approximately 99% of residential wastewater by weight is water, the task doesn’t seem so daunting. However, that 1% poses a threat to the environment and public health. In addition, we may equate wastewater with sewage, but it’s more than that. Water from showers, sinks, disposals and washers also needs to be treated, and can carry specific challenges. The components of wastewater can be characterized as organic and inorganic matter, oils and grease, nutrients, pathogens and solids.
Organic substances are carbon-based compounds – the building blocks of most living things. Organic materials in wastewater come from human waste, but also from paper products, foods, cosmetics and detergents. Most organic compounds are made of biodegradable proteins, carbohydrates or fats, which are easily broken down by microorganisms in the tank. The more organic material present in the wastewater, the more oxygen will be depleted to break these compounds down. This is reflected in the biochemical oxygen demand, which is a measurement of wastewater strength. BOD represents the amount of dissolved oxygen needed by aerobic biological organisms for breaking down organic material present in a water sample at a certain temperature over a specific period of time. When the time period is five days, the notation used will be BOD5. BOD is usually measured in milligrams per liter.
Inorganic minerals, metals and compounds, such as sodium, potassium and calcium, are common in wastewater from residential sources. Most inorganic substances are relatively stable and cannot be broken down easily by organisms in wastewater.
Oils and grease
Fatty organic materials from animals, vegetables and petroleum are also not quickly broken down by bacteria. On-site systems can be harmed by too much oil and grease, which can clog drain field pipes and soils, adding to the risk of system failure. Excessive grease also adds to the septic tank scum layer, which results in more frequent tank pumping.
Wastewater often contains large amounts of nutrients such as nitrogen and phosphorus. If the soil treatment area contains too much nitrogen, it may pass through to the groundwater and then surface water, which can lead to issues with algae blooms and loss of aquatic life. Denitrification systems may be integrated within the on-site system to manage nitrogen loading. Phosphorus originates from human waste, food residue and some cleaning agents, and can also cause issues with groundwater and surface water if loading is excessive.
Many disease-causing viruses, parasites and bacteria are also present in wastewater and can enter from almost anywhere in the community. Wastewater from typical homes contains enough pathogens to pose a risk to public health if not handled and treated properly.
Solid materials in wastewater include organic and/or inorganic materials and organisms. Solids will either settle, float or dissolve in the tank. Suspended solids that resist settling may pass through a tank and cause issues with clogging downstream. The measure of suspended solids is known as Total Suspended Solids and is usually measured in milligrams per liter.
Typical residential wastewater
The U.S. Environmental Protection Agency reports that the average daily wastewater flow from a typical residential dwelling is approximately 45 gallons per day. However, the American Water Works Association conducted a similar study on 1,100 households and found a median rate closer to 60 gallons per day.
The following table represents typical residential wastewater indicators in milligrams per liter.
According to a recent Water Environment Research Foundation study, the median TSS concentration in raw residential wastewater was 232 milligrams per liter. The median TSS concentration coming out of the septic tank was closer to 61 milligrams per liter.
Total nitrogen typically ranges between 20-to-85 milligrams per liter in untreated wastewater, and 50-to-90 milligrams per liter in septic tank discharge, so it can increase in the tank. 1
A septic tank is designed to recycle wastewater by gathering it from the source, treating it and dispersing it back into the natural environment. The final treatment stage usually involves soil in the dispersion area used in a mound or trench system, depending on the soil condition.
In the tank, several processes take place to provide partial treatment. These processes can be separated into three types: physical, biological and chemical.
Physical process in a septic tank
The tank operates as a flow attenuator because it serves to equalize the wastewater flow through the system. Large surges of flow from a residence – like when a toilet flushes or a washing machine drains – are dampened by a septic tank. This ensures the flow leaves the tank and enters the drain field at a lower rate and is extended over a long period of time. The tank will also help equalize the temperature of the wastewater when high-temperature flows enter the tank from household appliances.
The physical process that has the most impact on treatment within the tank is separation. Depending on a particle’s density and size as it enters the tank, it will either float, settle to the bottom or remain suspended in place. Most solids and oils will float or sink and create a top and bottom layer inside the tank, while a clearer middle layer of water will form, allowing the flow to move through to the next compartment or into the effluent filter. ASTM C1227, “Standard Specification for Precast Concrete Septic Tanks,” requires the transfer port on compartment walls be situated in the middle 25% of the water depth. As effluent moves to the second compartment, assuming the tank is designed this way, more settling and floating will occur while the clearer middle layer forms, allowing the flow to move into the outlet baffle through a filter and to the next treatment component. The key to the efficiency of this physical process is time, or more precisely, retention time. Substances in wastewater need time to float or sink. If a particle rushes through a tank in a series of velocity spikes, it won’t experience the effects of gravity. The tank must be designed to provide enough retention time for separation to occur.
Named after an Italian physicist, the Venturi effect describes how fluid increases in velocity when passing through a constriction – for example, as the wastewater in a tank is sucked through at a higher speed around a transfer port. ASTM C1227 states, “The transfer port between compartments shall be sized to maintain a low velocity as liquid moves between compartments. A minimum of 50 in.2 shall be used where local codes do not specify otherwise.” Using a larger hole or slot will slow down the velocity and increase the retention time and separation.
Let’s assume we have a two-compartment tank that is 4 feet wide inside and the water line is 4 feet from the bottom of the tank. The inlet and outlet pipe are 4 inches in diameter. Wastewater arrives through a .087 ft.2 pipe into a large, 16 ft.2 flow area, which leads to a .347 ft.2 pipe (if the tank is ASTM C1227 compliant) into another 16 ft.2 area and out through a .087 ft.2 pipe.
Keep in mind that the most retention time you’ll ever receive is when the tank is first used or after cleanings. As the bottom and top layers inside the tank get thicker, the middle layer gets thinner, resulting in increased velocities and less retention time. This supports the need for maintenance and cleaning on a regular basis.
Biological process in a septic tank
Let’s first define the term dissolved oxygen. This is the measurement of the amount of gaseous oxygen dissolved in the water. Oxygen enters the water through two natural processes:
- Diffusion from the atmosphere
- Photosynthesis by aquatic plants
Mixing surface water by wind and waves increases the rate at which oxygen can be dissolved or absorbed into the water. Adequate dissolved oxygen is important for water quality and necessary to all forms of life. Dissolved oxygen levels that drop below 5 milligrams per liter cause stress to aquatic life. This is why most fish tanks have an air pump. The bubbles increase the dissolved oxygen level, which ensures the survival of the fish.
Wastewater entering the septic tank contains measurable levels of dissolved oxygen. The microbial population in the tank rapidly depletes the dissolved oxygen as the flow disperses and moves toward the outlet. Previously, we discussed the three layers in septic tank wastewater. The middle zone is typically anoxic, which means there is little to no oxygen. In this region, microbes break down the biodegradable material. This process is called anaerobic digestion because it occurs without the need for oxygen. This anaerobic bacterial environment is the reason for the term septic.
This incomplete digestion produces methane, hydrogen sulfide and sulfur dioxide gases, and sludge. These microbes perform a very important function, and their growth and effectiveness depend on wastewater characteristics such as flow temperature; organic load; inorganic trash; toxic chemicals; cleaners and detergents; excessive fats, oils and greases; and pharmaceuticals and personal care products. This is why homeowner education on what not to put in septic systems is critical to system performance.
Chemical process in a septic tank
Chemical processes that occur in a septic tank include precipitation, adsorption and ion exchange. When a solution becomes a solid, it’s the result of a chemical reaction called precipitation. An example is when cations – positively charged calcium and magnesium – interact with anions such as negatively charged phosphate or ammonium to form crystals. Adsorption occurs when polar molecules react with organic molecules, binding compounds together.
What does this all mean?
We want to design our systems to provide enough retention time for proper initial treatment. The biggest impact a septic tank has on initial wastewater treatment is the level/concentration of TSS. It will also reduce BOD slightly. If there are circumstances that require additional treatment for lowering BOD, nutrients or pathogens, some options you can recommend or offer include advanced treatment, aerobic treatment, nitrogen removal and pathogen reduction. We will cover these extensively in the January-February issue of Precast Inc.
Do your research
There’s always room to enhance your ability to serve your customers. It takes research and time. A good way to conduct research is to attend wastewater conferences. From large national events to regional conferences, networking with suppliers and regulators will go a long way to help you understand the needs within your market and how to position yourself to supply those needs. This focus on enhancing your ability to provide solutions for a wide array of issues will benefits your company and clients for many years to come.
Claude Goguen, P.E., LEED AP, is NPCA’s director of sustainability and technical education.
1 Small & Decentralized Wastewater Management Systems, Crites and Tchobanoglous