Design of Wastewater Treatment Plants for Onsite Projects

Explore the pretreatment overview and learn how screening, the grease trap, and an equalisation tank remove solids, oil, and grease to protect downstream equipment.

Design coarse screening as the first unit, trapping large debris with bar screens to protect equipment. For small to medium flows, use 25–50 mm gaps and 30–45° slope.

Understand grease traps as a pre-treatment for food and beverage wastewater, removing fat, oil and grease to protect sewer lines and the primary treatment system.

Learn how an equalization tank buffers fluctuating wastewater flow, a pre-treatment step delivering a steady feed to primary treatment, using coarse bubble diffusers to prevent solids settling and odors.

Explore the primary treatment stage of sewage treatment, covering two non-septic methods. Learn how it removes fat, oil, grease, and suspended solids by gravity, with slight nitrogen and phosphorus reductions.

Explore how a two-compartment septic tank achieves primary treatment through sedimentation, flotation, and anaerobic digestion, with scum and sludge management and routine removal.

Design considerations for wastewater sedimentation tanks cover pipe diameters and placements, water depth 80–280 cm, two-compartment volumes, manholes 60×60 cm, and ventilation to control gases.

Design sedimentation chambers for a four-bedroom house with total volume 4.5 m³, V1 = 2 V2 (V2 = 1.5 m³, V1 = 3 m³), depth 1.6 m, width 1 m.

Maintain septic tanks with professional access and ventilation; monitor odors and crust, avoid additives, and schedule emptying every three years by checking sludge and scum depths.

The ABR, an anaerobic baffled reactor, upgrades septic tanks with vertical baffles for better sludge contact and COD removal, while energy-free and modular, requiring secondary and tertiary treatment for pathogens.

Maintain the NBA reactor by periodically removing scum and sludge, and keeping activated sludge at the bottom. Dislodging occurs every one to three years, depending on pre-treatment and tank design.

Design an ABR for onsite wastewater using five bathrooms, 3:4 width-to-length, 1–3 m depth, and 24–48 h hydraulic retention time; calculate digester width and use a multi-pipe tall outlet.

Design onsite wastewater plants by five cubic meters per day and a 48-hour retention time, yielding a 10 m3 four-compartment tank with 1.5 m depth and 2.4 m width, manholes.

Explore secondary wastewater treatment with aerobic and anaerobic microorganisms to reduce organic matter, nutrients, and suspended solids, and compare awts, mbbr, and constructed wetlands for onsite design.

Explain the design of aerated wastewater treatment systems as secondary treatment after septic or primary tanks, using air injection to form activated sludge and enable settling.

Size onsite wastewater treatment plants by calculating hydraulic retention time for isolation tanks (24-36 hours), sedimentation tank volume, and selecting air supply, diffusers, and blowers.

Design an activated sludge secondary treatment for a 10 m3/day flow, size two isolation tanks and a sedimentation tank to meet 20 mg/L effluent from 250 mg/L influent.

Introduce the moving beds biofilm reactor (mbbr) process with two isolation tanks and a sedimentation stage for bod and nitrogen removal, and explore the mbbr media, diffusers, and design values.

Design and performance of onsite wastewater systems, detailing body removal tank sizing, media volume, surface area loading rate, and aeration for nitrification and sedimentation.

Designs a two-stage MBBR secondary wastewater treatment with a primary septic tank and a sedimentation tank, sizing for 250 mg/L influent body to 10 mg/L effluent and NH3‑N 4 mg/L.

Explore horizontal flow constructed wetlands as a zero-energy, low-cost solution after a septic tank. Learn components, advantages, and performance targets, including 90% suspended solids removal and 99% E. coli reduction.

Size horizontal wetlands using the rule of thumb of five square metres per person, with gravity flow at 0.5–1 percent and a minimum six metre land length.

Perform routine visual checks of the HF wetland for weeds, plant health, and pests, and inspect inlet and outlet pipes for flow, blockages, water level, and outflow quality.

Design a horizontal flow wetland to treat onsite wastewater, calculating area and dimensions, using gravel layers, inlet/outlet protection, a water control system, and optional tertiary treatment or disinfection before reuse.

Outline the tertiary treatment after secondary treatment, focusing on disinfection to remove bacteria, viruses, and parasites, mainly using chlorination; filtration is needed for reuse but not for disposal.

Filtration follows secondary treatment in wastewater plants as a tertiary treatment, using sand and carbon filters to remove fine particles and pathogens, producing clear effluent suitable for reuse.

Learn how chlorine disinfection in onsite wastewater plants removes pathogens after secondary treatment, using chlorine gas, sodium hypochlorite, or tablets, and manage chlorine demand and residual chlorine for safety.

Uv disinfection uses a uv lamp to inactivate bacteria, viruses, and protozoa by damaging nucleic acids at 254 nanometers, a rapid, chemical-free physical process after secondary treatment.

Assess treated wastewater reuse for irrigation, highlighting nutrients nitrogen, phosphorus, potassium, health risks, treatment levels, and guidelines from WHO, FAO, and US EPA, plus irrigation methods and regulations.