Impact of Wastewater Effluent on Rivers and the Use of Reclaimed Water Supplies

L.R. Chevalier, Ph.D., P.E., D-WRE, BCEE, F-ASCE

Curriculum for Sustainability at Southern Illinois University CarbondaleBased onChevalier, L.R., 2010, Impact of Wastewater Effluent on Rivers and the Use of Reclaimed Wastewater Supplies, Center for Sustainable Engineering, http://www.csengin.org/library.htm

Objectives Investigate the basic operations of a waste water

treatment facility Find sources of information on the characteristics

and per capita generation of wastewater Review the basic calculations for the input

parameters of the DO Sag model (Streeter-Phelps) Evaluate the impact of releasing wastewater

effluent into a river using the DO sag model Identify the issues involved with the direct and

indirect use of treated wastewater as a municipal water supply source

Initial Activities Tour a water reclamation (waste water treatment)

facility Draw a schematic of the facility and describe the

treatment objective of each unit process. Include an estimate of the daily flow as well as the facility’s capacity.

Use a technical resource to describe five major characteristics of wastewater. Develop a glossary that defines the terms reported.

Identify two by-products of wastewater treatment. What are the characteristics and uses of these by-products? Can these by-products be reduced or used commercially?

Virtual Tour: District of Columbia District of Columbia Water and Sewer Authority Largest advanced wastewater treatment plant in the world

Capacity of 370 million gallons per day (MGD) Peak capacity of 1.076 billion gallons per day and Covers 150 acres.

To collect wastewater 1,800 miles of sanitary and combined sewers 22 flow-metering stations, 9 off-site wastewater pumping stations, and 16 stormwater

pumping stations  Separate sanitary and storm sewers serve approximately two-

thirds of the District of Columbia In older portions of the system, such as the District's downtown

area, combined sanitary and storm sewer systems are prevalent.

Blue Plains Wastewater Treatment Plant

O Street Pumping Facility

Virtual Tour

Elmhurt, IL WWTP

Background: Growth Rate Models

ktePP 0

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

500

1000

1500

2000

2500

3000

3500

Time (days)

Popu

latio

n

Activity: Determine which curve is for k= 0.5 day-1. What is the rate constant for the other curve? Discuss how you arrived at your answer. The initial population for this example is 150.

Background: Mixing 1st container

2 L container with a concentration of

100 mg/L. Determine the mass

(mg) of the contaminant in the container by multiplying the concentration by the volume

(100 mg/L)(2L) = 200 mg

Background: Mixing

2nd container 0.5 L 500 mg/L.

The mass of the contaminant in this container (500 mg/L)(0.5 L) = 250 mg

Background: Mixing

Mix the two containers. Total volume of water = 2 L + 0.5 L =

2.5 L Total mass of contaminant = 200 mg +

250 mg = 450 mg The final concentration uses these

combined calculations, noting that concentration is mass divided by volume: Final concentration = 450mg/2.5 L =

180 mg/L

Background: Mixing

One stream is flowing at a rate of 0.5 L/s with a concentration of 2000 mg/L.

he other stream is flowing at a rate of 1.5 L/s with a concentration of 200 mg/L.

We will now consider the calculations needed to determine the concentration in the combined stream.

Q1 C1

Q2 C2

Q3 C3

Background: Mixing

Mass flux: mass/time [M/T] For the first stream

(0.5 L/s)(2000 mg/L) = 1000 mg/s For the second stream

(1.5 L/s)(200 mg/L) = 300 mg/s Mass Flux for the combined streams

(M/T)1 + (M/T)2 = (M/T)3

1000 mg/s + 300 mg/s = 1300 mg/sQ1 C1

Q2 C2

Q3 C3

Background: Mixing

Add the flows Q1+Q2 = Q3

0.5 L/s + 1.5 L/s = 2.0 L/s Final concentration

(1300 mg/s) ÷ (2.0 L/s) = 650 mg/L

Q1 C1

Q2 C2

Q3 C3

DO Sag Model

Dissolved oxygen (DO) in river water is the source of oxygen used by aquatic life.

DO sag model is used to evaluate whether wastewater effluent released into a stream will cause the dissolved oxygen in the stream to go below levels needed for a healthy stream.

The evaluation starts by considering the river water at the point of the effluent discharge.

DO Sag Model

EFFLUENT FROM A WATER RECLAMATION PLANT

DO Sag Model

Calculate the initial dissolved oxygen in a stream at the point of wastewater effluent release

Using the subscript “s” for the stream, “eff” for the effluent and “t” for total (QsCs + QeffCeff)/Qt = Ct

where Qt = Qs + Qeff

DO Sag Model

Activity: Find a reference for the dissolved oxygen concentrations needed for different species of river fish and other aquatic life. Discuss whether there is more biodiversity at higher or lower dissolved oxygen concentrations.

Dissolved Oxygen Deficit, D D = DOsat – DOt Da = DOsat – DOinitial

Activity: Find a reference for dissolved oxygen concentrations for a range of water temperatures above freezing and below boiling. Discuss the reason why the dissolved oxygen concentration changes. Would you expect the same trend for other dissolved gases or dissolved solids? Justify your answer.

DO Sag Model

𝐷= 𝑘𝑑𝐿𝑎𝑘𝑟 − 𝑘𝑑ሺ𝑒−𝑘𝑑𝑡 − 𝑒−𝑘𝑟𝑡ሻ+ 𝐷𝑎𝑒−𝑘𝑟𝑡

0 1 2 3 4 5 6 7 80123456789

10

Time (day)

Dis

solv

ed O

xyge

n (m

g/L)

DO sat

D

Ultimate BOD, La

BOD is the Biochemical Oxygen Demand Reported as a concentratrion

(QsBODs+QeffBODeff)/Qt = BODt = La

𝐷= 𝑘𝑑𝐿𝑎𝑘𝑟 − 𝑘𝑑ሺ𝑒−𝑘𝑑𝑡 − 𝑒−𝑘𝑟𝑡ሻ+ 𝐷𝑎𝑒−𝑘𝑟𝑡

0 1 2 3 4 5 6 7 80123456789

10

Time (day)

Dis

solv

ed O

xyge

n (m

g/L)

DO sat

D

Rate Constants, k kr ,rate of reaeration or reoxygenation of the

stream kd ,rate of deoxygenation. Rate constants are dependent on the system

under investigation and on temperature.

0 1 2 3 4 5 6 7 80123456789

10

Time (day)

Dis

solv

ed O

xyge

n (m

g/L)

DO sat

D 𝐷= 𝑘𝑑𝐿𝑎𝑘𝑟 − 𝑘𝑑ሺ𝑒−𝑘𝑑𝑡 − 𝑒−𝑘𝑟𝑡ሻ+ 𝐷𝑎𝑒−𝑘𝑟𝑡

Critical time, tc

𝑡𝑐= 1𝑘𝑟−𝑘𝑑𝑙𝑛ቈ𝑘𝑟𝑘𝑑ቆ1−𝐷𝑎𝑘𝑟−𝑘𝑑𝑘𝑑𝐿𝑎 ቇ

0 1 2 3 4 5 6 7 80123456789

10

Time (day)

Dis

solv

ed O

xyge

n (m

g/L)

DO sat

Mimimum dissolved oxygen, DOmin

Maximum deficit, Dmax

Critical time, tc

Spreadsheet Model

Spreadsheet Model

Activity: There are a number of input parameters for the DO sag model. To understand the impact or sensitivity of a given parameter on a system under study, investigate a range of values of one parameter while keeping all other values constant. Generate a graph that shows how this value impacts the minimum DO or tc. Discuss what values of the model can be controlled by the operation of the treatment plant.

0 1 2 3 4 5 6 7 80123456789

10

Time (day)

Dis

solv

ed O

xyge

n (m

g/L)

DO sat

DOmin

DOsat -Da

Question of Sustainability In this exercise, you have investigated the

fundamental aspects of wastewater treatment, and the impact of effluent release into a river.

As the demands of water supplies increase, and in some regions of the world critically scarce, the direct reuse of waste water is a discussion of value.

In fact, wastewater treatment is being recognized as water reclamation in many discussions.

Question of Sustainability

Activity. Read Babcock et al, 2004, Chen and Wang, 2009, Shoenberger and Sorgini 2009. Discuss their findings and the how it relates to the issue of water sustainability. Find additional papers that discuss the issue.

References Environmental Protection Agency, Sustainable Infrastructure for

Water and Wastewater, http://www.epa.gov/waterinfrastructure/bettermanagement_energy.html

Babcock, R.W., McNair, D.A., Edling, L.A. Nagato, H., 2004, Evaluation of a system for residential treatment and reuse of wastewater, J. Environmental Engineering-ASCE, 130(7):766-773.

Chen, R., Wang, X.C, 2009, Cost-benefit evaluation of a decentralized water system for wastewater reuse and environmental protection, Water Science Technology, 59(8):1515-1522.

Shoenberger, P., Sorgini, L., 2009, Solving potable water shortages with wastewater reclamation, Water and Wastes Digest, http://www.wwdmag.com/Solving-Potable-Water-Shortage-with-Wastewater-Reclamation--article7591.

NomenclatureBOD, biochemical oxygen demandDa, initial oxygen deficitDO, dissolved oxygenDOsat, saturated dissolved oxygenk, rate constantL, literLa, biochemical oxygen demand at point of sewage dischargeM, massmg, milligramQ, flow s, secondt, time