Discuss the aspects of a chemical flocculation process design that must be considered during the engineering process.

Upon completion of this unit, students should be able to:

1. Assess the fundamental science and engineering principles applicable to the management and treatment of solid and hazardous wastes.

1.1 Discuss the techniques of coagulation, flocculation, and sedimentation as they relate to an engineered precipitation process for wastewater treatment.

1.2 Describe the decision making to either increase the pH or decrease the pH of the wastewater treatment system in order to effectively precipitate heavy metals.

5. Evaluate operations and technologies related to industrial and hazardous wastes.

5.1 Discuss the aspects of a chemical flocculation process design that must be considered during the engineering process.

5.2 Discuss the aspects of a secondary circular clarifier process design that must be considered during the engineering process.

Reading Assignment

Chapter 3:

Chemical Treatment

Unit Lesson

In this unit, we are going to learn about the technology available to us in Bahadori’s (2014) discussion on chemical treatment. As such, we are going to continue with our industrial and hazardous waste treatment system design by adding chemical treatment and disinfection processes into our system.

The chemical treatment and biological treatment of the waste influents are often considered to be two of the most challenging aspects of the entire treatment system. This is due largely to the fact that chemistry and biology are statistically reliable to an average of about 95%. This means that the other five percent of the time the anticipated chemical and biological activity related to a reaction (chemical or enzymatic) may not work as forecasted. In fact, this is why it is common for us as scholar-practitioners of environmental engineering to conduct a chemical and biological hypothesis level of 95% (Trochim, 2001). We must remember that we are actually testing in research and design (R&D) activities with a statistical confidence, attempting to manipulate nature in order to effectively separate solids (metals and organic materials), gases (volatiles and semi-volatiles), synthetic liquids (organic and halogenated solvents), and water (Texas Water Utilities Association [TWUA], 1991). This is often very challenging. This is why we turn to technological solutions for many of these process options.

Chemical treatment and biological treatment are causally related variables within the treatment system. In fact, both the chemical and biological treatment processes have the ability to causally affect the other in tandem (Haas & Vamos, 1995). Stated another way for clarity, the chemical treatment process often informs the biological treatment process. Additionally, the biological treatment process can also inform the chemical treatment process. For example, we may effectively reduce the biochemical oxygen demand (BOD) during the chemical treatment process, but then experience still another BOD change in the process with the interruption of aerobic organisms’ enzymatic activities, such as the catalase enzyme described by Bahadori (2014). Consequently, it is very important for us as engineers to closely consider the chemical and biological treatment processes as dynamic processes, rather than static processes inherent during physical treatment (such as oil removal). Let’s start with the chemical treatment process. MEE 5801, Industrial and Hazardous Waste Management 2

UNIT x STUDY GUIDE Title

Bahadori (2014) describes the chemical treatment process in terms of subsystems (e.g., precipitation, coagulation, chemical oxidation and reduction). However, he does not order these subsystems in a way that is necessarily easy for us to understand in terms of industrial and hazardous waste treatment. Instead, let’s consider the following subsystems in this specific order of treatment techniques, with the correlating principles (Haas & Vamos, 1995; TWUA, 1991) tied to each subsystem within chemical treatment (Bahadori, 2014):

1. Neutralization (pp. 82-83, 93)

a. pH (acid/alkaline) and ion exchange

b. acid waste neutralization (NaOH, NaOCl)

c. alkaline waste neutralization (H2SO4, HNO3)

2. Chemical oxidation and reduction (pp. 82-84)

a. ion exchange (mineral softener unit, ion exchange column)

b. cyanide reduction (alkaline chlorination, O3, or H2O2 treatment)

c. activated carbon adsorption (liquid phase granular absorber)

d. air and steam stripping (stripping column or distillation tower)

3. Precipitation (pp. 81-82, 87-94)

a. coagulation (Ca(OH)2, Al2(SO4)3, FeCl3, in tandem with polyelectrolytes)

b. flocculation (cationic polyacrylamide)

c. secondary hydroxide precipitation (NaOH, Ca(OH)2)

4. Solidification and stabilization (pp. 84, 90-92)

a. clarifying (secondary clarifier tank)

b. sludge thickening (cationic polymers)

c. sludge dewatering (filter or belt press)

5. Disinfection (pp. 94-98)

a. chemical agent disinfection (chlorination)

b. mechanical agent disinfection (filtration)

c. biological agent disinfection (activated sludge and Unit IV techniques)

In order to facilitate the chemical reactions associated with neutralizing and reduction/oxidation (redox reactions) activities, it is common to use an inverted cone shaped vessel or Imhoff tank, named after Dr. Karl Imhoff (TWUA, 1991) in order to facilitate greater solids collections into the hopper-shaped bottom while using the gravimetric techniques of flocculating solids (sedimentation) from the liquid phased waste solution (Haas & Vamos, 1995).

It is often in the Imhoff tank that one of the most critical aspects of the entire chemical treatment process can occur. This is the precipitation of heavy metals from the solution. Through the manipulation of the wastewater pH (neutralizing the pH from very low or very high values), the neutralization process actually initiates the redox reaction process. In turn, the redox reactions initiate the precipitation process and subsequently the solidification process. The heterogeneous equilibria of the solution becomes very important in this process, given that the equilibrium constant could be defined as the following (Haas & Vamos, 1995):

This means that we could actually then define the solubility of a heavy metal by estimating the activity of a hydroxide ion (OH), and subsequently determine the required equilibrium pH of the wastewater at which a select metal would precipitate (Haas & Vamos, 1995). It is by this method that we realize most heavy metals tend to precipitate within a higher pH wastewater matrix (TWUA, 1991). MEE 5801, Industrial and Hazardous Waste Management 3

UNIT x STUDY GUIDE Title

Consequently, one effective way to tie together all of these subsystems into a single chemical treatment process could be the following order of in-line equipment, following the physical oil removal equipment:

Oil-water separator (Unit 2) Imhoff tank (neutralization/precipitation)

ion exchange column (oxidation/reduction) secondary

clarifier (stabilization) filter press (solidification)

Now, let’s consider our second phase of equipment needs (chemical treatment) for our proposed project design as we use our interactive model again.

1. Click here to access the interactive design model.

2. Closely review the influent laboratory report (lift station) against the effluent laboratory report (pop up report). Remember that the goal is to design our system so that the final effluent concentrations meet the established local limits for the municipal wastewater treatment plant (WWTP).

3. Continue to use this model in your design work for your course project (proposed industrial and hazardous waste treatment facility) again in this unit.

Have fun designing your chemical treatment process within your industrial and hazardous waste treatment system!

References

Bahadori, A. (2014). Waste management in the chemical and petroleum industries. West Sussex, United Kingdom: Wiley.

Haas, C., & Vamos, R. (1995). Hazardous and industrial waste treatment. Upper Saddle River, NJ: Prentice-Hall.

Texas Water Utilities Association. (1991). Manual of wastewater treatment. (6th ed.). Austin, TX: Author.

Trochim, W. M. K. (2001). The research methods knowledge base (2nd ed.). Cincinnati, OH: Atomic Dog.

Suggested Reading

The suggested reading will give you additional resources related to the content for this unit. The article is an open source publication and can be found at the URL provided below

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