Drinking Water Treatment

An Introduction

Eckhard Worch

• Publisher: De Gruyter
• Year: 2019
• ISBN:
  9783110551662
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

This publication provides the scientific fundamentals for understanding chemical, physical and biological processes that are used in drinking water treatment, such as filtration, coagulation, softening, deironing, demanganization and others. Written in a compact and easily accessible form, the book is focused on the objectives, the theoretical basics and the practical implementation of the treatment processes.

1 Introduction

1.1 Natural and urban water cycle

Water is the basis of all life. For humans, water is essential for survival and therefore irreplaceable. Ensuring water availability and quality belongs to the most important goals of sustainable development. The quality of water is determined by its constituents, which is the totality of the substances dissolved or suspended in water. Although water with a total mass of about $1.38\times {10}^{18}\mathrm{t}$ is the most common molecular substance on Earth, its distribution between the individual environmental compartments is strongly unbalanced (Table 1.1). Approximately 97% of the total water is salt water of the oceans that cannot be consumed by humans directly. Although the production of drinking water from ocean water is, in principle, possible (e.g., by reverse osmosis), it is unfavorable due to the high energy demand for the necessary treatment processes. Therefore, freshwater is typically used as the raw water resource for drinking water production. The most important freshwater resources are the polar ice caps and the glaciers as well as groundwater and surface water. From these resources, only parts of surface water and groundwater can be utilized to produce drinking water with an acceptable technical effort. It is estimated that less than 1% of the huge water resource is actually available for human use.

Tab. 1.1Global water resources. Data from Trenberth et al. (2007).

Water resource	Volume (${10}^3{\mathrm{km}}^3$)	Portion (%)
Oceans	1 335 040	96.95
Polar ice caps and glaciers	26 350	1.91
Groundwater	15 300	1.11
Lakes and rivers	178	0.013
Soil moisture	122	0.009
Permafrost	22	0.0016
Atmosphere	12.7	0.0009
Total	1 377 024.7	100.0

However, the usable freshwater inventories are constantly renewed by the hydrological water cycle. Figure 1.1 shows the global water cycle in a very simplified form. Approximately $\mathrm{400\; 000}{\mathrm{km}}^3$ of water are transferred by evaporation from the sea into the atmosphere each year and the same volume is returned by precipitation to the mainland and to the oceans. The cycle thus resembles a distillation plant whose energy demand, covered by the sun, can be calculated from the enthalpy of vaporization of water to be about ${10}^{21}\mathrm{kJ}/\mathrm{a}$. For the renewal of freshwater resources, the proportion of evaporated water from the oceans that is transported through the atmosphere to the land is of particular importance. At $\mathrm{40\; 000}{\mathrm{km}}^3/\mathrm{a}$, this proportion is approximately one tenth of the water evaporated from oceans.

Fig. 1.1 The global hydrological water cycle. Volumetric flow rates in ${10}^3{\mathrm{km}}^3/\mathrm{a}$. Data from Trenberth et al. (2007).

A part of the rainwater is added directly to the surface water bodies (streams, rivers, lakes, reservoirs) or flows on the land surface into the surface water. Another part of the precipitate, the seepage water, infiltrates into the soil. There it is absorbed by plants or further transported into the ground where it finally reaches the groundwater level. Surface water can also infiltrate into the subsurface where it becomes groundwater. After more or less long residence times and flow paths, groundwater returns to the surface in the form of springs or directly by exfiltration into surface water. The hydrological cycle is completed by evaporation processes and water runoff to the sea. The evaporation can take place both from the ground and from the water surfaces as well as from plants. The runoff to the sea occurs via creeks, rivers, and streams.

Despite the fact that only water is evaporated from the oceans and the dissolved substances remain as ‘distillation residual’ within the aqueous phase, naturally occurring freshwater water is never pure ${\mathrm{H}}_2\mathrm{O}$ but contains dissolved, colloidal, and coarsely dispersed constituents that are introduced into the water in the different stages of the hydrological cycle by natural processes and anthropogenic activities. Nevertheless, the concentrations of the major components (i.e., inorganic ions) are much lower in freshwater than in ocean water. The highest ion concentrations in freshwater are in the $\mathrm{mg}/\mathrm{L}$ range, whereas the highest ion concentrations in seawater are in the $\mathrm{g}/\mathrm{L}$ range.

In contrast to seawater, with a relatively constant composition, the composition of natural freshwaters varies strongly. It is influenced by a variety of physical, chemical, and biological processes, which often act in a very complex manner inside the water phase but also at the boundaries with other environmental compartments such as the atmosphere or solid phases like soils or sediments. Therefore, the composition of freshwater not only varies depending on the type of the water body (e.g., groundwater, lakes, rivers,...