Wastewater Treatment: Characteristics and Systems

Significance of Wastewater Contaminants Suspended solids – can cause sludge deposits and anaerobic conditions in the environment Biodegradable organics – can cause anaerobic conditions in the environment Pathogens – transmit disease Nutrients – can cause eutrophication Heavy metals – toxicity to biota and humans Refractory organics – toxicity to biota and humans Dissolved solids – interfere with reuse

Characteristics of Wastewater Physical Colour – grey Septic sewage is black (due to precipitation of iron sulfide) Chemical Number of chemicals found in w/w is limitless Measure of w/w strength often use COD and BOD COD – Chemical Oxygen Demand BOD – Biological Oxygen Demand

Characteristics of Domestic Wastewater

On-Site Disposal Systems In locations where sewers and a centralized wastewater treatment system are not available, on site disposal must be used Septic systems most common for individual residences “Engineered systems” used for unfavorable site conditions Larger systems required for housing clusters, rest areas, commercial and industrial facilities

Septic Systems

Septic Systems Septic Tank – settling, flotation and anaerobic degradation

Septic Systems Drain field (cross-section) – aerobic degradation

Septic Systems Soil must pass percolation test Design specifications soil type rate of water infiltration depth to water table Design specifications Tank volume and number of chambers Drain field size Drain field materials Basis for design is empirical Tank must be “pumped” to remove solids every 1-3 years Drain field replacement may be required

Engineered Systems Engineered system required for soils that don’t “perc” – common in many areas, especially lakefront property in MI Typical approach is to design and build something equivalent to a drainfield Mound systems most common in MI Other types of “filters” may be used Typically about 3 times more expensive than drain field

Engineered Systems Mound System

Engineered Systems Intermittent sand filter can be designed for pulsed dosing even distribution high treatment efficiency leakage protection

Municipal Wastewater Treatment Systems Pretreatment – removes materials that can cause operational problems, equalization optional Primary treatment – remove ~60% of solids and ~35% of BOD Secondary treatment – remove ~85% of BOD and solids Advanced treatment – varies: 95+ % of BOD and solids, N, P

Pretreatment of Industrial Wastewaters Industrial wastewaters must be pretreated prior to being discharged to municipal sewer system Approach is to remove materials that will not be treated by municipal system Local authority must monitor and regulate industrial discharges Pretreatment requirements set by U.S. EPA

Bar racks Purpose Solid material stored in hopper and sent to landfill remove larger objects Solid material stored in hopper and sent to landfill Mechanically or manually cleaned

Grit Chambers Purpose: remove inert dense material, such as sand, broken glass, silt and pebbles Avoid abrasion of pumps and other mechanical devices Material is called “grit”

Grit Chambers: Velocity Controlled

Type I Settling -- Stokes’ Law where νs = settling velocity ρs = density of particle (kg/m3) ρ = density of fluid (kg/m3) g = gravitational constant (m/s2) d = particle diameter (m) μ = dynamic viscosity (Pa·s)

Example: Grit Chamber Design Design a grit chamber to remove sand particles (p = 2650 kg/m3) with a mean diameter of 0.21 mm. Assume the sand is spherical and the temperature of the wastewater is 20 oC. The wastewater flow is 10,000 m3/d. A velocity of 0.3 m/s will be automatically maintained, and the depth must be 1.5 times the width at maximum flow. Dynamic viscosity: mPa-s Pa-s=10-3mPa-s Pa=N/m2 N=kg-m/s2 Therefore, Pa-s = (kg-m/s2)(1/m2)(s) = kg/(m-s)

Example Calculate settling velocity

Example Calculate the cross-sectional area

Example Calculate the width and depth

Example Determine the detention time required for a particle to fall the entire tank depth Determine the length to achieve this detention time

Example Thus, the tank must have dimensions W = 0.51 m D = 0.76 m L = 5.8 m