Exam 4 Study Guide

 What % of the US uses on-site wastewater systems?

25%

What % of the NC population uses on-site wastewater systems?

50%

 How many on-site systems are installed each year in NC?

15,000

Average septic effluent concentrations of total nitrogen

60 mg/L

 Average septic effluent concentrations of total phosphorus

8.1 mg/L

Average septic effluent concentrations of fecal coliform

10⁶ CFU/100mL

Five major components of on-site wastewater systems

1. Septic tank

2. Distribution box

3. Drainfield trenches

4. Soil under trenches

5. Setback distances

Septic tank

A watertight container that holds wastewater long enough for solids to settle to the bottom, forming sludge and oils/grease to float to the top, forming scum. Anaerobic bacteria start to break down organic matter before the effluent exits the tank. Organic nitrogen is converted to ammonia and precipitation of phosphorus minerals occurs. 

Distribution box

A small box that receives effluent from the septic tank through an inlet and divides it equally among multiple drainfield trenches through multiple outlets. Equal distribution ensures no single trench becomes overloaded. 

Drainfield trenches

Underground corrugated pipes laid in gravel-filled or gravelless trenches that disperse the effluent evenly into the surrounding soil. They store wastewater temporarily while it slowly infiltrates into the ground. 

Soil under trenches

Acts as the final treatment for wastewater. Filters out pathogens, traps suspended solids and allows aerobic microbes to break down any remaining organic matter and nutrients before the water reaches groundwater. 

Setback distances

Mandatory horizontal buffer zones required between system components and surrounding features like wells, surface waters, property lines, and building foundations. They prevent wastewater contamination of drinking water and protect public health. 

 Physical treatment mechanisms in soil

- Filtration = wastewater is drawn into smaller pores first; helps remove some pollutants and bacteria

- Sedimentation = small soil pores allow fine particulate matter to settle out onto soil surfaces

- Dilution = mixing wastewater with cleaner water → pollutants are still there   

- Dispersion = direction and how it moves away from the site

 Chemical treatment mechanisms in soil

- Cation exchange = NH4+ can stick to exchange sites in the soil

- Adsorption of P

- Precipitation = Dissolved minerals like P reaction with iron, aluminum, or calcium in the soil to form insoluble solid compounds that are locked in place

Biological treatment mechanisms in soil

- N-Transformations = nitrification in aerobic conditions and denitrification in anaerobic conditions (organic matter and near water table) 

- Predation by native soil microorganisms 

- Oxidation

What method is used in the field to determine soil particle size distribution?

Texture by feel method

What method is used in the lab for determining particle size distribution?

Hydrometer method

Minimum soil depths for permitting a shallow conventional

24-30 inches

Minimum soil depths for permitting a LPP

20-24+ inches

Minimum soil depths for permitting a low profile chamber

20-26 inches

Minimum soil depths for permitting a mound system

18 inches but also Seasonal high-water table must be deeper than 12 inches

What does LTAR stand for?

Long Term Acceptance Rate

Why do sandy soils have higher LTAR’s than clay soils?

Sandy soils have higher LTAR’s than clay soils because they consist of larger particles that create wide pore spaces aka macropores that allow water to drain rapidly. This means sandy soils can absorb a greater volume of effluent per square foot each day. 

Why would a pump system be necessary for some sites?

- Necessary for some sites when gravity alone cannot move wastewater from the septic tank to the drainfield. For example, if the drainfield must be installed at a higher elevation than the septic tank.

- It is also necessary when pressurized distribution is required to spread effluent evenly across the soil. Pressurized distribution is required in large drainfields and specialized systems, such as the low pressure pipe.

What are the benefits of using a chamber and polystyrene trench media relative to a conventional with gravel media?

- Polystyrene trench media = less labor intensive to install and 25% reduction in drainfield area

- Chamber system = More storage area, less labor intensive to install, and 25% reduction in drainfield area

Benefits of using an LPP

Uniform effluent distribution, suitable for shallow soils, and soil texture doesn’t matter

Disadvantages of using an LPP

Requires pump tanks and an operator to maintain the system twice a year → Maintenance agreement required, and hard to install 

Two disease organisms that are resistant to low doses of chlorine

Cryptosporidium parvum and Giardia lamblia  

Septic System Design Step 1 - Determine flow

a. 120 gallons per day per bedroom

b. Multiply # bedrooms by 120 gpd

c. units = g/d

Septic System Design Step 2 - Based on soil type and soil group, assign long term acceptance rate (LTAR)

a. Determine texture of soil, and then place into soil group (I-IV)

b. Pick a loading rate within the range for the soil group

c. If mound system, then you must use lowest LTAR in the range for the

soil group

d. Max LTAR for panel block is 0.8 gpd/ft2 in Group I soils

e. Max LTAR for 25% reduction product is 1.0 gpd/ft²

Septic System Design Step 3 - Determine area of trench bottom required

a. Divide flow by LTAR

b. units = ft²

Septic System Design Step 4 - Determine linear footage of trench needed

a. Divide LTAR by width of trench (typically 3 ft)

b. If polystyrene or chamber, then multiply area by 0.75 (25%

reduction)

c. If Bed system then multiply area by 1.5

d. If panel block multiply area by 0.5 (50% reduction)

Septic System Design Step 5 - Length of each trench and overall area of drainfield

a. Divide the linear footage of a trench by the number of total trenches → length of each trench

b. Total width of the trenches = whatever width of the trench x 3 = distance between trenches from the center and then multiply this value by the number of centers between trenches and add 3 inches to account for the two 1.5 inches of space before and after the trench centers at the start and end

c. List the dimensions in ft

How to draw an Onsite Septic System

- House will be 60 ft long and 30 ft wide

- Driveway will be 12 ft wide

- Septic tank = 5 ft wide & 10 ft long

- Trenches = draw a line, must be 9 ft apart; should be the length determined from calculations; run out of room → can draw trenches parallel to each other and bend/curve them → must equal the calculated length

Important Setbacks for drawing an Onsite Septic System

- Septic tank should be 5 ft apart from the house

- Setback of 10 ft on all sides of whole septic system

Primary Treatment

Uses several mechanical processes are to remove the larger suspended solids through screening and sedimentation; 50-65% of suspended solids are removed and BOD (biochemical oxygen demand) is reduced

Secondary Treatment

Depends on biological processes. Microorganisms, predominately aerobic bacteria, break down organic materials. Uses two methods:

- Trickling filters = beds of crushed stone whose surfaces are covered with a microbial slime consisting of bacteria, protozoan, nematodes, etc. that absorb organic material

- Activated sludge process = Consists of “seeding” (microbes grow in anaerobic conditions) a tank of sewage with bacteria-laden sludge, pumping compressed air into the mixture and agitating for 4-10 hours

Tertiary Treatment

Biological nutrient removal; Involves the microbial transformation of dissolved organic nitrogen through a series of steps to un-reactive elemental nitrogen (N₂).

- Only required when: treated wastewater will be used for the purposes of drinking, irrigation, or recreation

Approximately how many people die each day due to fecal contamination of drinking water?

Almost 5,500 people die every day 

Four basic steps for water purification used at most large water treatment plants

- Sedimentation

- Coagulation

- Filtration

- Disinfection

Sedimentation

Incoming raw water settles for 24 hours in a tank for heavy sediment to settle out

Coagulation

Alum {Al₂(SO₄)₃, NaAlO₂} is added to water to precipitate out smaller solids in the form of flocs. Helps small particles stick together.

Filtration

Uses media, such as beds of sand or diatomaceous earth that have a large surface area that can filter out solids 

Disinfection

Kills pathogens in the water. Chlorine is most commonly used and leaves residuals that continue to kill microbes.

 Factors that influence chlorine disinfection efficiency

1. Concentration of interfering substances = (Can reduce disinfection rate)

2. Contact Time (15+ minutes suggested)

3. pH (pH lower than 7.4 is best)

What are some other water disinfection techniques/processes?

Ultraviolet Light and Ozone