Environmental Engineering: Fundamentals and Design - Chapter 8

BOOK TABLE OF CONTENTS
CHAPTER TABLE OF CONTENTS
TOC > Ch. 8 > 8.2 River water quality

# Chapter 8 Water Quality

Martin T. Auer, Alex S. Mayer, James R. Mihelcic and Noel R. Urban

## River water quality

### River Dissolved Oxygen Simulator

Introduction

Engineering is about design. In river water quality management, as with the companion module on lake water quality, design often calls for the estimation of Total Maximum Daily Loads (TMDLs), i.e. the maximum quantity of a material that may be discharged to a receiving water without violating water quality standards. This particular case considers the discharge of BOD from a wastewater treatment plant (WWTP) to a river and the resulting impact on the dissolved oxygen (DO) sag curve. This analytical approach to the maintenance of a sustainable ecosystem has been known since the work of Streeter and Phelps on the Ohio River in the 1920's. Today, computer-based simulations are more sophisticated and complex, but follow the same principles.

Objective

Rivers receiving discharges of oxygen demanding substances are vulnerable to episodes of oxygen depletion (Section 8.2.1) with attendant negative impacts on aquatic biota (Figure 8.2). Here, a wastewater treatment plant is being designed to discharge its effluent to a river. A TMDL for BOD, as reflected in a plant BOD removal efficiency (%) will be calculated such that the river water quality standard for DO (5 mg L-1) is not violated under critical conditions of flow and temperature.

About the Model

The model framework applied here is that of the classic Streeter-Phelps approach (Section 8.2.4). The model performs a mass balance on oxygen in a river simulated as a plug flow reactor (PFR, Section 4.1.5), i.e.

where O2 is the oxygen concentration (mg•L-1), t is time (d), k2 is the reaeration rate coefficient (d-1), D is the oxygen deficit (mg•L-1), k1 is the in-stream deoxygenation rate coefficient (d-1) and L is the ultimate CBOD of the waste (mg•L-1). Additional details of the model are provided in Section 8.2.

In the application developed here, the user will develop a TMDL for BOD discharged in the effluent of the wastewater treatment plant by calculating the percentage removal required to meet a water quality standard. The sensitivity of the calculation to environmental conditions and the efficacy of alternate, supplemental management actions will also be explored.

Management Considerations

The river is most vulnerable to the discharge of oxygen-demanding wastes when the flow is diminished (increasing the mixing basin CBOD and decreasing the mixing basin DO) and temperature is elevated (increasing the CBOD rate coefficient and reducing the oxygen content at saturation). Thus the TMDL is established for these critical conditions. Secondary treatment of municipal wastewater can routinely remove 80% of the influent CBOD, but higher efficiencies (in excess of 90%) are possible at greater cost. As a means of mitigating capital and operating costs, it may be advantageous to consider supplemental management options such as alternative discharge points (e.g. sites with greater potential for reaeration), industrial pre-treatment (to reduce the biodegradability of the influent), post-aeration (addition of oxygen to the effluent) and large scale receiving water manipulation (e.g. flow augmentation).

Instructions

LAUNCH the River DO Simulator in its own window.

The River DO Simulator consists of a CALCULATION worksheet and a SUMMARY worksheet. Simulations are explored in the CALCULATION worksheet. The results are stored as a set in the SUMMARY worksheet.

Use the River DO Simulator to complete each of following Scenarios. At the end you can explore your own treatment scenarios.

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SCENARIO 1: Annual Average versus Critical Conditions

Conditions

Model inputs for annual average conditions are pre-populated on the spreadsheet as AA. Under critical conditions, the river flow falls to 1.5 m3 s-1 and the temperature rises to 30°C. All other model inputs are as for average annual conditions. Critical conditions are represented as CC. Use Trial as an adjustable working trial.

Analysis/Questions

Observe the simulated DO sag curves and data for AA and CC with a treatment efficiency of 80%.

1. Is the dissolved oxygen standard violated for annual average conditions (AA) with the present level of wastewater treatment?
2. Is the dissolved oxygen standard violated for critical conditions (CC) with the present level of wastewater treatment?
3. RESET Trial to Critical Conditions. Adjust the BOD Removal efficiency until the water quality standard for DO is met: 5 mg L-1. Record the BOD Removal efficiency.

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SCENARIO 2: Addition of Post-Aeration

Conditions

Unless special provisions are made, the effluent may leave the WWTP with very low oxygen levels (e.g. the 2 mg L-1 of the critical conditions scenario). Oxygen can be added to the effluent (post-aeration) to increase DO levels in the mixing basin and reduce adverse effects on the receiving water.

Analysis/Questions

RESET Trial to Critical Conditions. Apply post-aeration treatment: WWTP Effluent DO = 8 mg L-1. Adjust the BOD Removal efficiency until the water quality standard for DO is met. Record the BOD Removal efficiency.

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SCENARIO 3: Addition of Industrial Pre-Treatment

Conditions

Municipal wastewater may contain industrial wastes (e.g. from food processing) that are highly biodegradable and thus elevate the influent BOD rate constant. Pre-treatment of the industrial waste prior to discharge to the municipal system can reduce the biodegradability of the waste.

Analysis/Questions

RESET Trial to Critical Conditions. Apply industrial pre-treatment: reduced BOD rate constant k1 = 0.15 d-1. Adjust the BOD Removal efficiency until the water quality standard for DO is met. Record the BOD Removal efficiency.

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SCENARIO 4: Reaeration

Conditions

The reaeration rate constant varies with the turbulence of the river, which may, in turn change among river reaches as the depth and bed shape change. It may be most favorable to site the effluent discharge in a reach exhibiting a higher reaeration rate constant.

Analysis/Questions

RESET Trial to Critical Conditions. Apply reaeration adjustment due to a site change: higher aeration rate constant k2 = 0.50 d-1. Adjust the BOD Removal efficiency until the water quality standard for DO is met. Record the BOD Removal efficiency.

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SCENARIO 5: Design Recommendations

Conditions

Design engineers have available the suite of engineered options examined in the previous scenarios. It remains to develop a recommendation for a package of engineered alternatives that will economically meet the water quality standard.

Analysis/Questions

RESET Trial to Critical Conditions. Apply the combinations of engineered alternatives outlined below and record the required BOD removal efficiency for each.

Compare the results for the three alternatives as recorded in the SUMMARY. Develop a final recommendation, balancing the efficacy of a particular alternative with the feasibility of its implementation.

A. In-system alternatives: selection of a discharge location takes into account factors in addition to water quality impacts, e.g. design of the wastewater collection system. In making a final recommendation, it is desirable to first consider in-system alternatives that would allow the proposed site to be utilized. Determine the required BOD removal efficiency for the proposed site with industrial pre-treatment plus post-aeration. Record the BOD removal efficiency.

B. Alternate discharge location: application of in-system alternatives may not yield a required treatment efficiency that is reasonably achieved. In this case, an alternate location on the same receiving water that offers more assimilative capacity (reaeration) may be considered. Determine the required BOD removal efficiency with increased aeration (Scenario 4) plus industrial pre-treatment and post-aeration. Record the BOD removal efficiency.

C. Flow augmentation: in cases where the limits of treatment technology are approached and where excess water is available, low flows may be augmented by water from storage reservoirs. This serves to dilute the effluent discharge, reducing the BOD in the mixing basin. Determine the amount of flow augmentation (m3 s-1) required to meet the water quality standard for an easily achievable BOD removal efficiency (80%) under critical conditions. Record the augmented river flow.

In some cases this alternative may not be feasible. For example, augmentation may not be a viable solution where there are competing demands for the water (e.g. irrigation, drinking water supply) or where the required amount of water is simply not available. Convert the flow requirement to units of millions of gallons per day (1 m3 s-1 = 22.8 MGD) and compare this demand with the amount of water available from a given reservoir in New York City reservoir system. Would flow augmentation be feasible?

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There is no correct answer here. The path forward is a matter of the economics and efficacy of in-plant alternatives, access to alternate discharge locations and the availability of a water supply for flow augmentation. Sometimes it's important to know when to quit. If the expense associated with plant design and operation is prohibitive for a proposed receiving water, it may be best to identify a different receiving water or an alternative method of disposal such as water reclamation.

Explore two alternatives for water reclamation and re-use:
surface application and subsurface injection.

#### Credits

River DO Simulator: Copyright 2008 Michigan Technological University.