Chevalier, 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 SUSTAINABILITY CURRICULUM Module: Impact of Wastewater Effluent on Rivers and the Use of Reclaimed Water Supplies Level: Sophomore/junior environmental engineering or environmental science Author: L.R. Chevalier, Ph.D., P.E., D-WRE, BCEE, F-ASCE Professor, Dept. of Civil and Environmental Engineering, Southern Illinois University Carbondale The overall objective of this module is to introduce students to the basic concepts of waste water treatment, the impact of releasing treated waste water into a stream or river, and some of the issues involved with the sustainable reclamation of water. In this module, the student will be required to find and use technical references that provide information on water and wastewater. Therefore, tabular reference data and schematics typically found in environmental engineering text and reference books will not be presented. SPECIFIC LEARNING OBJECTIVES 1. Investigate the basic operations of a waste water treatment facility 2. Find sources of information on the characteristics and per capita generation of wastewater 3. Review the basic calculations for the input parameters of the DO Sag model (Streeter-Phelps) 4. Evaluate the impact of releasing wastewater effluent into a river using the DO sag model 5. Identify the issues involved with the direct and indirect use of treated wastewater as a municipal water supply source INITIAL ACTIVITY 1. Tour a water reclamation (waste water treatment) facility 2. 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. 3. Use a technical resource to describe five major characteristics of wastewater. Develop a glossary that defines the terms reported. 4. 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?2 | P a g e BACKGROUND GROWTH RATE MODEL Growth and decay of microorganisms can be described by a basic population growth rate model (1) where Po is the initial population, P is the population at time t and k is the rate constant. The rate constant is an important parameter for understanding the treatment of wastewater. As you discovered on your tour of a wastewater treatment facility, microorganisms are used to reduce the organic loading of wastewater. Microorganism growth can be estimated using an exponential growth model. For example, consider a batch reactor with bacteria and wastewater containing organics. The initial number of bacteria is 150. Figure 1 shows how the bacteria will multiply over time using two different rate constants. FIGURE 1: POPULATION GROWTH RATE MODEL 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. MIXING FLUIDS WITH DIFFERENT CONCENTRATION When estimating concentrations of a mix from two or more sources, we cannot directly add concentration. Instead, we must convert the concentration to mass, then add the mass. Consider the following problem. We have two containers of water that have different concentrations of a pollutant. Now combine the two containers, and estimate the concentration in the final mix. In this example, we will use a common unit of concentration, mg/L (milligrams per liter). Let’s start by considering the first container, which is a 2 L container with a concentration of 100 mg/L. First determine the mass (mg) of the contaminant in the container by multiplying the concentration by the volume. 05001000150020002500300035000 1 2 3 4 5Population Time (days)3 | P a g e (100 mg/L)(2L) = 200 mg Now consider a 0.5 L container with the concentration of 500 mg/L. The mass of the contaminant in this container is: (500 mg/L)(0.5 L) = 250 mg If we mix the two containers, we need to consider the total volume of water and the total mass of the contaminant. 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 Now let’s consider how we could calculate this if we have water flowing instead of in a container. For this example, we will consider the point where two streams meet. But also note that when a treatment plant releases treated water (or effluent) into a river, we also need to make this calculation. The letter Qj will represent flow, which is volume over time. In our example, we will use liters per second, L/s. The letter Cj will be used for concentration, which mass over volume. In our example, we will use milligrams per liter, mg/L. Consider the following data for our example. One stream is flowing at a rate of 0.5 L/s with a concentration of 2000 mg/L. The 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. As in the previous example, this is done by considering the mass. Because the mass is moving with flowing water, we will calculate mass flux, which means mass over time. (0.5 L/s)(2000 mg/L) = 1000 mg/s FIGURE 2: GENERALIZED SCHEMATIC OF STREAMS CONNECTING4 | P a g e Now let’s consider the second stream (1.5 L/s)(200 mg/L) = 300 mg/s The next step involves adding the two flows and the two masses, giving a total flow and a total mass. In order to track our calculations, subscripts will be used. First, consider water flow Q1+Q2 = Q3 0.5 L/s + 1.5 L/s = 2.0 L/s Now consider mass flux: (M/T)1 + (M/T)2 = (M/T)3 1000 mg/s + 300 mg/s = 1300 mg/s To convert this to a concentration of mg/L, use the total water flow and total mass flux. (1300 mg/s) ÷ (2.0 L/s) = 650 mg/L DO SAG MODEL Dissolved oxygen (DO) in river water is the source of oxygen used by aquatic life. The DO sag model is