wastewater midterm 1

wastewater

liquid wastes and wasted transported in water from households, commercial establishments, and industries, as well as storm water and other surface runoff; may contain high concentrations of organic and inorganic pollutants, pathogenic microorganism, as well as toxic chemicals

time line of wastewater

• open dumping

• collection & open dumping

• collection and disposal without treatment

• collection and disposal after treatment

• collection and treatment for resource recovery and reuse

suspended solids removal options

• coarse screens, fine screens

• grit chamber

• clarification

• filtration

• chemically enhanced clarification

colloidal and dissolved solids removal

• chemical precipitation

• membrane filtration

• ion exchange

• activated carbon adsorption

biodegradable organics removal options

• suspended growth processes

• attached growth processes

• ponds and lagoons

• membrane bioreactors

pathogens removal options

• chlorination

• ozonation

• ultraviolet disinfection

nutrients and nitrogen removal options

• biological nitrification-denitrification

• air stripping

• breakpoint chlorination

phosphorus removal options

biological phosphorus removal

chemical precipitation

volatile organic compounds removal options

• activated carbon adsorption

• air stripping

preliminary and primary treatment steps

wastewater influent→screens→comminutor→grit chamber→primary clarifier

secondary treatment steps

biological reactor → secondary clarifier

sludge treatment steps

gravity thickener → anaerobic digester → centrifuge

clean water act

• 1972 and 1977 amendments

• regulating discharge of pollutants

• regulating water quality standards

emerging wastewater treatment issues

• rising energy costs

• disposal of biosolids

• performance and reliability of plants

• presence of endocrine disrupting compounds and toxic chemicals in wastewater (PFAS, micro and nano plastics)

• scarcity of fresh water

future directions for WW treatment

• energy generation: electricity and thermal energy generation from anaerobic digesters

• beneficial reuse of biosolids: reuse as fuel and fertilizer

• ww reuse

• fundamental research and mathematical modeling

reaction kinetics

study of the effects of temp, pressure, and concentration of reactants and products on the rate of a chemical reaction

chemical reactor

tank or vessel where chemical, biological, or biochemical reactions take place, usually in a liquid medium; chemical reactions are the major transformation mechanism

ideal reactors

batch reactor; plug flow reactor (PFR); continuous-flow stirred tank reactors (CSTR)

conversion/removal efficiency

( [A₀] - [A_t] ) / [A₀]

material balance

(rate of input) = (rate of output) + (rate of accumulation) - (rate of consumption)

batch reactor

time period for reaction begins just after the reactor is filled and ends just before contents are emptied → rate of input = 0 = rate of output
- rate of consumption = rate of accumulation

plug flow reactor

rate of input = rate of output - rate of consumption (at steady state)

continuous flow stirred tank reactor

rate of input = rate of output - rate of consumption (at steady state)

batch reactor process

• materials added to the reactor and mixed for a period of time necessary for the chemical reactions to occur

• mixture is removed at the end of the reaction

• all fluid elements have the same residence time in the reactor

• at any instant, the reactor contents are homogeneous and have a uniform composition

batch reactor uses

• liquid phase reactions

• bench scale experiments

batch reactor cons

• not suitable for commercial scale applications, especially for gas phase reactions

• labor costs and material handling costs are high

PFR steps

• elements of homogeneous fluid move through the reactor tube as plugs moving parallel to the tube axis

• fluid elements discharged in the same sequence as they entered

• no longitudinal mixing although lateral mixing may be there

• concentration of reactant across any vertical cross section is the same

• all fluid elements have same residence time

PFR uses

• suitable for continuous gas phase reactions that take place at high pressure and temp

• heat transfer is minimized by putting an insulating jacket around the reactor

• no moving parts inside reactor

• average reaction rate is higher in a PFR as compared to a CSTR with the same volume (except for 0 order rxns)

CSTR steps

• reactants flow continuously into the reactor and the product stream is discharged on a continuous basis while the reactor contents are continuously mixed

• influent reactant concentration is immediately reduced to final effluent concentration

CSTR uses

• simple to construct and easy to control

• ease of access to interior surface

• mainly used for liquid phase reactions at low or atmospheric pressure

series of CSTRs design equation

r_a = ( [A]_i - [A]_(i-1) ) / t_i

semi-batch or semi-flow reactor

• batch reactor partially filled with one reactant with progressive addition of other reactants till reaction completion

• reactiants added at same time but products removed continuously

• reactants added intermittently

• products removed intermittently

self-purification mechanisms of natural water

physical, chemical, and biological processses

self-purification factors

• hydraulic characteristics: volume, rate, turbulence of flow

• physical characteristics: bottom and bank material, variations in sunlight and temp

• chemical nature of the natural water

physical processes

dilution, sedimentation and resuspension, filtration

dilution

• success depends upon discharging relatively small quantities of waste into large bodies of water

sedimentation

• sources of suspended solids: domestic ww, industrial ww, runoff from ag activities

• solids may be organic, inorganic, or live organism

• size from large to colloids

in suspension solids increase turbidity which leads to

• reduced light penetration → restricts photosynthetic activity of plants

• inhibits vision of aquatic animals

• interferes with feeding of aquatic animals that obtain food by filtration

drawbacks of sediment deposits

• anaerobic conditions develop and any organics trapped in them will decompose → soluble compounds in the stream

• alter the streambed by filling up pore space → creates unsuitable conditions for reproduction of many aquatic organisms

• reduce reservoir storage capacities

• increase flooding due to channel fill in

filtration

small particles of organic matter or inorganic clays may be filtered out by rocks along the streambed

as water percolates from surface downward into groundwater if the soil layers are deep and fine enough, removes suspended materials

sunlight photolysis

sunlight can directly excite chemicals and degrade them and can also excite dissolved organic matter to produce reactive oxygen species for chemical oxidation

microbial degradation

• bacteria can utilize contaminants as carbon sources or nutrients and degrade them

• metabolic and co-metabolic pathways

• oxidation, reduction, hydrolysis

phytoremediation

• in-site restoration process that uses plants to transform, immobilize, and extract contaminants from soils or water

objectives of ww treatment

• reduce level of solids, biodegradable organic matter, pathogens, and toxic compounds to meet regulatory limits

infiltration and inflow

includes water that eventually enters the sewer from foundation drains, leaking pipes, submerged manholes, and groundwater infiltration

preliminary treatment

• removal of larger suspended solids and inert materials, physiscal processes

• unit operations: screens, comminutors/grinders, grit chambers, flow equalization

flow of prelim treatment

trash rack → bar screen → fine screen → comminutor → grit chamber

grit

particle having a d = 0.2mm with a G_s = 2.65 and settling veloctiy = 0.3 m/s

determination of design flow rate depends on

• design period (20-25 years)

• population prediction

• estimation of wastewater flow

• estimation of infiltration and infow

• variability of ww flow

screens

• placed ahead of pumps and used to preclude entrace of materials into treatment plant

trash racks

• screens with large openings to exclude larger debris and garbage

• consist of rectangular or circular steel bars arranged in parallel fashion with openings 5.1-5.2 cm

coarse or bar screens

• clear openings 2.5-7.6cm

• manually cleaned screens should be place at angle

• mechanically cleaned can be placed vertically

comminutors

• shredding devices, grind solids ¼ to 3/8 in

grit chamber types

• horizontal flow: channel shaped settling tanks with an effluent weir

• aerated: air keeps lighter organics in suspension while heavier grit settle to bottom

• vortex: vortex flow pattern generated