Course: BFM 203 Water Treatment and Control

Lecture

The Water Cycle
Credits
CHANGE

BFM 202 Biodiesel Production Unit 1 Lecture has been adapted from "ENGY 131 Unit 1 Commentary: What is biodiesel?" by Southeastern Illinois College.

ENGY 131 Biodiesel Production course content by Southeastern Illinois College is licensed under a Creative Commons Attribution 3.0 Unported License. Except where indicated otherwise.

Unit 1: Water Sources, Types of Treatment Processes, and Operator Duties and Responsibilities

Where does all the Earth's water come from? Primordial Earth was an incandescent globe made of magma, but all magmas contain water. Water set free by magma began to cool down the Earth's atmosphere, and eventually the environment became cool enough so water could stay on the surface as a liquid. Volcanic activity kept and still keeps introducing water into the atmosphere, thus increasing the surface and groundwater volume of the Earth.

Water Cycle

All water comes in the form of precipitation, but 92 percent either evaporates immediately or eventually runs off into the ocean. Water evaporation from the ocean is caused by energy and the sun. The water that evaporates is salt free. Water vapor rises, is carried by winds, and eventually condenses into clouds. When the clouds cool, vapor forms into water droplets and precipitates over land or water. As water falls in the form of rain, snow, sleet, or hail, it clings to dust, dirt, and other contamination in the air. A large part of falling water makes it to the ground where it becomes part of our fresh water supply. This water either soaks into the ground or runs off, but all water eventually returns to the ocean.

What is the water cycle? As you may remember from WQM100, the water cycle describes the existence and movement of water on, in, and above the Earth. Earth's water is always in movement and is always changing states, from liquid to vapor to ice and back again. The water cycle has been working for billions of years and all life on Earth depends on it continuing to work; the Earth would be a pretty stale place without it.

A quick summary of the water cycle

Here is a quick summary of the water cycle. The links in this paragraph go to the detailed Web pages in the USGS water site for each topic. Each link opens in a new browser window. A shorter summary of each topic can be found further down in this page, though.

The water cycle has no starting point, but we'll begin in the oceans, since that is where most of Earth's water exists. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air; a relatively smaller amount of moisture is added as ice and snow sublimate directly from the solid state into vapor. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil. The vapor rises into the air where cooler temperatures cause it to condense into clouds.

Air currents move clouds around the globe, and cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt. Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff, and groundwater seepage, accumulate and are stored as freshwater in lakes.

Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration. Some of the water infiltrates into the ground and replenishes aquifers (saturated subsurface rock), which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge, and some groundwater finds openings in the land surface and emerges as freshwater springs. Yet more groundwater is absorbed by plant roots to end up as evapotranspiration from the leaves. Over time, though, all of this water keeps moving, some to reenter the ocean, where the water cycle "ends" ... oops - I mean, where it "begins."

Source: http://ga.water.usgs.gov/edu/watercyclesummary.html

Surface Water

Surface water is generally characterized by soft water, turbidity, suspended solids, color, and microbial contamination. Surface water accumulates mainly as a result of direct runoff from precipitation. Precipitation that does not enter the ground through infiltration or return to the atmosphere by evaporation is considered direct runoff. In colder climates, snow accumulates in the mountains in the winter and becomes runoff as temperatures begin to warm in the spring. Surface water supplies are divided into rivers, streams, lakes, and reservoirs.

Lakes and Reservoirs

Ponds, lakes, and open reservoirs are at higher risk for contamination and the consequent spread of disease. Water from lakes and reservoirs is unsafe for domestic use without proper treatment, which includes filtration and disinfection. The quality of this water sources is subject to seasonal changes due to stratification and turnover. What is stratification? This term refers to the formation of separate layers in a lake or reservoir. Each layer has similar characteristics throughout the layer such as temperature or dissolved oxygen level. Water from lakes and reservoirs is susceptible to algal blooms, especially after turnover.

Lakes may be used recreationally, but limitations may need to be imposed to reduce the risk of contamination. Limitations may include 1) no gasoline motors, 2) no direct contact (swimming), 3) no overnight camping, and/or 4) no fishing.

Treatment plants on large reservoirs often use a multi-port intake structure (a structure with gates positioned at different levels) that can draw water from the dept of best quality. The lowest level intake is not normally used for treatment but instead for draining the reservoir. The uppermost intake may not be used regularly because of low reservoir levels. The image below is of a multi-level intake system that is under construction.

Rivers and Streams

When utilizing water from rivers and streams, we must always be concerned with upstream conditions. Any pollutants are likely to draw downstream from treated wastewater discharge, agricultural drainage, or industrial waste. The water is also susceptible to scouring of the bottom, changing channels, and silt. Any design for intake structures should take into consideration flooding, heavy silting, icing, and adverse runoff conditions. Wildfires and de-forestation may have an adverse effect on the amount and type of material that exists in the runoff into rivers and streams. Constant monitoring of the water source is necessary.

(Photos courtesy of the Red Rocks Community College Water Quality Program)