5 Wastewater Characteristics

CE 356: Fundamentals of Environmental Engineering

Wastewater Characteristics

Ricardo B. Jacquez

Professor, CAGE Department

New Mexico State University

Teaching Assistant: M. T. Myint

Learning Objectives

• Comprehend basic characteristics of domestic wastewater (flow rate, BOD, SS) and apply these characteristics to a preliminary engineering design.

• Calculate mass loadings within the wastewater treatment system for BOD and SS.

• Calculate and interpret mass balances for mixtures of wastewaters.

• Calculate and interpret population equivalents for industrial wastewater.

Sources of Wastewater (Major Components)

1. Domestic: food, soap and detergents, bathroom (fecal and urine), and paper.

2. Commercial: bathroom and food from restaurants and other “stores.”

3. Industrial: highly variable, dependent on industry, controlled by pre-treatment regulations.

4. Runoff from streets: sand and petroleum and tire residues (infiltration, not a direct discharge).

Wastewater Characteristics

Characteristics are categorized as:

- Physical

- Chemical

- Biological

Concentrations characterize wastewater as:

weak, medium, and strong.

Physical Characteristics

• Solids

- Total solids: residue after evaporating all water

(50% volatile and 50% fixed)

(Volatile=organic. Fixed=inorganic mg/L)

- Suspended solids: residue retained on a filter

(75% volatile and 25% fixed).

- Dissolved solids: residue passing through a

filter (40% volatile and 60% fixed).

- Settleable solids: residue after one hour settling in an

Imhoff cone (ml/L) (the indicator of primary sludge).

“SEWAGE”

• Domestic wastewater is the ideal mixture of biodegradable nutrients and microorganisms:

– BOD (organics)– Nitrogen (N)– Phosphorus (P)– Miscellaneous Micronutrients (S, Fe, Ca, . . . )– Bacteria

Chemical Characteristics

• Organics: Predominant biodegradable fraction, BOD

-Protein -Fats, Oils, and Greases

-Carbohydrates -Surfactants (detergents)

-Urea

• Priority pollutants – Industrial solvents, pesticides, etc.– Pre-treatment standards control

concentrations to trace levels

Chemical Characteristics cont.

• Inorganic Matter

– Nitrogen: from protein and urea.

NH2, NH3, NH4+ forms ammonia and organics.

An essential nutrient for growth of bacteria.

– Oxygen Demand:

NH4+ + 2O2 = NO3

- + 2 H+ + H2O

Chemical Characteristics cont.

• Phosphorous: phosphate (PO4-3)

– Inorganic-detergents– organic-food and metabolic wastes

• Trace nutrients: S, Fe, Mn, Mg, Ca, K, Zn, Mo, etc. . . . . . plentiful

Mass Loading Calculations

• Mass = Conc x Flow Rate

= mg/L x MGD x 8.34 x 1/24

= lbs/hr

• Mass = Conc x Flow Rate

= mg/L x ft3/sec x 0.22

= lb/hr

Mass Balance Calculations

• Fundamental Law: Massin = Massout

• Massin = Q1 x Conc1 + Q2 x Conc2 (knowns)

• Massout = QT x ConcT

(unit constants cancel out)

• QT = Q1 + Q2

• ConcT = unknown

Biological Characteristics

“Most” microorganisms known to microbiologists can be found in domestic wastewater.

- Bacteria

- Protozoa

- Viruses

- Algae (limited)

Population Equivalents

1. Wastewater discharge: 100 -120 gpcd

- 70 gpcd domestic

- 10 gpcd industrial/commercial

- 20 to 40 gpcd infiltration

2. Suspended Solids and BOD

SS = 0.2 lb pcd (without kitchen grinder)

SS = 0.22 lb pcd (with kitchen grinder)

BOD = 0.17 lb BOD pcd

BOD = 0.26 lb BOD pcd (with kitchen grinder)

Population Equivalents cont.

For 100 gpcd and consider for one person:

BOD0.17 lb

100gal

1

8.34 lb

106 galmg

L

BOD 204mg

L

SS0.2 lb

100gal

1

8.34 lb

106 galmg

L

SS 240

mg

L

Population Equivalents cont.

• Used to – Establish population equivalency of an

industrial waste.– Establish charge for treating industrial waste

based on BOD or SS rather than flow.

Find the BOD and flow equivalent for an industrial waste with the following characteristics:

Q 0.1MGDMGDMGD BOD 450mg

L

Flow: 0.1MGD

100gal

person day

1000peoplepeoplepeople

BOD: 0.1MGD 450 mg

L

8.34 lb

MGDmg

L

0.17 lb BOD

person day

2208peoplepeoplepeople

BOD equivalent is 2.2 times greater than flow equivalent.