Environmental Biotechnology - Jordening and Winter

49

2

Industrial Wastewater Sources and Treatment Strategies

Karl-Heinz Rosenwinkel, Ute Austermann-Haun, and Hartmut Meyer

2.1

Introduction and Targets

This chapter deals with the wastewater flows discharged from industrial plants and offers a synopsis of the applicable treatment methods.

The central topic of this book being biotechnology, the main emphasis of this chapter is on industries emitting wastewater with organic pollutants, since these can be treated biologically. Next, the possible wastewater flow fractions occurring in industrial plants are listed. According to each type of industry and individual local conditions, certain wastewater flow fractions do not occur, are disposed of, or are treated in a different manner. Then, various wastewater pollutants are investigated with regard to their direct importance for certain industries. This is followed by a typical treatment sequence for wastewater. In the main section, wastewater composition and possible treatment strategies for the individual industries are examined, with particular emphasis on the most important branches of the food processing industry.

We should note that there are not only substantial differences between the various industrial branches, but that even within one branch wastewater composition and appropriate treatment methods are determined by the following factors:

•production methods

•water supply and water processing

•technical condition and age of the production site

•training and motivation of employees

•use certain additives and cleaning agents, etc.

•number of shifts, seasonal differences (campaign operation)

•effluent requirements (direct or indirect discharge)

•extent of production-integrated environmental protection means

•number of wastewater treatment facilities

Environmental Biotechnology. Concepts and Applications. Edited by H.-J. Jördening and J. Winter Copyright © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30585-8

502 Industrial Wastewater Sources and Treatment Strategies

2.2

Wastewater Flow Fractions from Industrial Plants

2.2.1

Synopsis

Due to the multitude of products and production methods in industrial plants, there is also a wide range of different wastewater flow fractions for which the respective industrial branches have coined their own particular terms (e.g., ‘singlings’ is a term used only in the distillery business). The wastewater flow fractions listed below represent the most important wastewater sources, without each fraction being necessarily produced by each single branch:

•rainwater

•wastewater from sanitary and employee facilities

•cooling water

•wastewater from in-plant water preparation

•production wastewater (the following flow fractions are in some branches considered part of the production water):

–wash and flume water

–fruit water

–condensates

–cleaning water

2.2.2

Rainwater

Rainwater can periodically constitute a substantial proportion of wastewater, depending on the amount of sealed surface. It is important to differentiate between nonpolluted to slightly polluted rainwater, such as that coming from roofs, which should be percolated or can be discharged directly into the storm sewer or waterways, and rainwater that is collected from sealed areas where product handling or vehicles contribute to contamination of rainwater, making treatment of rainwater necessary.

The amount of rainwater QR (in L s–1) can be calculated by the following formula:

QR = rD,n · ØS · AE

(1)

where AE (in ha) is the catchment area, i.e., all areas that channel runoff into the rainwater canal in the event of rainfall. ØS (dimensionless) is the runoff coefficient, which indicates the percentage of fallen rainwater that actually increases the runoff flow (i.e., the portion that does not percolate or evaporate or is not held back by lowlying areas). This amount varies greatly, depending on the surface conditions of the catchment area. For sealed industrial areas it is safe to use a runoff coefficient ØS = 1. rD,n represents the rainfall intensity in L s–1 ha–1. For a 10-min rainstorm (with a sewer flow time of >10 min, the standard design rainfall intensity), which

2.2 Wastewater Flow Fractions from Industrial Plants 51

in this intensity occurs only once a year, this value amounts in Germany to 100–200 L s–1 ha–1. If sewer overflow is not permissible once a year, but only once every five years, the given amounts need to be increased by a factor of 1.8. A detailed design capacity for the rainfall amount can be found in ATV (1992).

The nature of the pollutants and the resulting pollutant load in the rainwater depend mainly on the prevailing degree of air and surface pollution. According to ATV (1992), the average COD value in rainwater runoff is 107 mg L–1. Peak values, which occur especially after long dry periods at the onset of a rainfall, are substantially higher.

2.2.3

Wastewater from Sanitary and Employee Facilities

Wastewater from sanitary and employee facilities consists of the water used by the employees for washing and for flushing toilets and the wastewater from other employee facilities. If there is a large cafeteria, the wastewater from this source should be evaluated separately.

Wastewater from sanitary and employee facilities has the same basic composition as domestic wastewater and should therefore be kept separate from the effluent emerging from the production line. This flow fraction should be discharged into the municipal sewage system, but treatment in the factory’s own treatment plant is also possible.

The amount of wastewater per employee and shift is approx. 40–80 L, but the condition of the plumbing fixtures and the degree of dirt involved in the operation (e.g., use of showers at the end of a shift) can have a major impact on the amount of wastewater produced. The load per employee and shift can be assumed to be 20–40 g BOD5, 3–8 g N, and 0.8–1.6 g P.

2.2.4

Cooling Water

If cooling is necessary and done with water, one must differentiate between continuous flow (high water consumption) and recirculation (increase in salt concentrations).

No general amounts can be given for the quantity of cooling water considered necessary, because the need for cooling water varies greatly from one industry to the next. As a first approach, one can say that, for industries using recirculation cooling towers, approx. 3%–5% of the water capacity (depending on the salt concentration in the cooling water, this ratio may even be higher) needs to be extracted to prevent excessive increase in pollutant concentrations.

If there are no leaks in the a continuous-flow cooling tower and no chemical additives are used in the cooling water, the composition of the water does not change, but its temperature increases. In recirculation cooling towers the concentration of pollutants increases with evaporation of the cooling water.

522 Industrial Wastewater Sources and Treatment Strategies

2.2.5

Wastewater from In-plant Water Preparation

Wastewater from in-plant water preparation can result from the preparation of drinking water from well water or from water softening, decarbonation, or desalination systems. The contents of this water depend on the characteristics of the raw water, of the treatment method(s) used, and of the chemicals used.

2.2.6

Production Wastewater

Production wastewater is a generic term, which includes a variety of flow fractions. Exact specifications can be made only in relation to a particular industry. Certain flow fractions arising in specific industries have been designated with a particular name (e.g., ‘singlings’) and are not listed here. The following wastewater flow fractions are either investigated as a separate flow fraction or as part of the production wastewater:

•Produce washing water is also used in many food processing industries as flume water for transport of the produce.

•Fruit water is the term used in the food processing industry for that water that is extracted from the processed fruits and vegetables, such as the water extracted from potatoes during starch production.

•Condensates are the exhaust vapors that have been condensed after having been removed during an evaporation or drying process.

•Cleaning water results from washing the production lines (pipes, containers, etc.), cleaning the production facilities, and cleaning the transportation containers (bottle washing).

2.3

Kinds and Impacts of Wastewater Components

Below, the most important wastewater components are listed, with their influence on the value retention of the production facilities and their impact on the necessary wastewater treatment measures.

2.3.1

Temperature

The temperature has a major influence on the construction material. High temperatures are not suitable for synthetic materials; e.g., they may cause damage to gaskets. Thus, the temperature of wastewater that is discharged into municipal sewage systems is restricted (in Germany the standard discharge temperature is <35 °C). With regard to the corrosion of metallic materials, the temperature is a major factor, next to the chloride and oxygen contents. Another aspect is that volatile components are easily removed from the wastewater and volatilized at higher temperatures.

2.3 Kinds and Impacts of Wastewater Components 53

An increased wastewater temperature has a positive influence on biological wastewater treatment methods, since an increase in temperature also increases the activity of microorganisms. On the other hand, any temperature increase results in a lower oxygen input capacity of the aeration system.

2.3.2 pH

Similar to the temperature, the pH value has an impact on determining the suitable construction materials and the activity of microorganisms. To avoid damage to the sewage systems and the connected treatment plants, in Germany the pH value of the discharged effluent should be between 6.5 and 10. In treating wastewater, it is of importance which factors have influenced the pH. A low pH, for example, might result from organic or inorganic acids. Inorganic acids should be neutralized; organic acids should be biologically removed.

For many microorganisms the ideal living conditions are at a relatively neutral pH value. Anaerobic wastewater treatment processes are especially sensitive to fluctuations in pH. It is important to keep in mind that organic acids can, at suitable organic loads, be degraded by biological plants without having to be neutralized. This, however, does not apply to mineral acids.

2.3.3

Obstructing Components

With regard to wastewater treatment, obstructing components are large inorganic particles, such as glass shards, plastic parts, cigarette butts, sand, etc., which are biologically inert and need to be removed mechanically to avoid damage, clogging, or caking in the subsequent treatment process.

2.3.4

Total Solids, Suspended Solids, Filterable Solids, Settleable Solids

The content of solid particles has a substantial effect on the amount of organic matter. One differentiates between total solids (TS), which consist of suspended solids (SS, all particles that do not pass through a membrane filter with a pore size of 0.45 ìm), and filterable solids (FS). Settleable solids (unit: mL L–1) are the solids that will settle to the bottom of a cone-shaped container; therefore, they do not include all of the suspended and floating solids.

2.3.5

Organic Substances

Organic substances constitute the main pollutant fraction in most industrial plants. To prevent direct oxygen consumption in the waterways into which the effluent is discharged, organic substances need to be eliminated as far as possible. A variety of

54 2 Industrial Wastewater Sources and Treatment Strategies

parameters can be used to determine the content of organic matter in a given wastewater sample: BOD, COD, TOC, DOC, etc. These sum parameters, however, do not include any indication of the kind of organic substances measured.

The COD has developed into a major parameter, because the results of COD analysis are available much more quickly than those of BOD analysis. The use of cuvette tests gives an excellent cost–benefit ratio and requires less effort, space, and time to obtain results.

The COD/BOD ratio is an important value in determining the biodegradability of the pollutants in a particular wastewater. If the ratio is <2, the load is considered easily biodegradable.

2.3.6

Nutrient Salts (Nitrogen, Phosphorus, Sulfur)

Nutrient salts are inorganic salts such as NH4, PO4, SO4, which are considered vital for the growth of plants and microorganisms. Since nitrogen and phosphorous can cause massive growth of biomass and may, therefore, lead to eutrophication of waterways into which the effluent is discharged, in the European Union nitrogen and phosphorous must be eliminated before wastewater is discharged into sensitive waterways.

To eliminate nitrogen and phosphorous biologically, sufficient organic pollutants must be present. Therefore, not only is the concentration of these substances in the industrial wastewater important, but also the ratio of their concentrations to the COD or BOD.

Some kinds of industrial wastewater have such low nitrogen and/or phosphorous concentrations that nitrogen (in the form of urea) and/or phosphorous (as phosphoric acid) must be added to obtain the necessary minimum nutrient ratio for growth of the microorganisms needed for biodegradation.

2.3.7

Hazardous Substances

Hazardous substances are a generic term used for those substances or substance groups contained in wastewater which must be regarded as dangerous because they are toxic, long-lived, bioaccumulative, or have a carcinogenic, teratogenic, or mutagenic impact. In industrial wastewater, the following substances are of major importance:

•absorbable organic halogen compounds (AOX)

•chlorinated hydrocarbons and halogenated hydrocarbons

•hydrocarbons (benzene, phenol, and other derivatives)

•heavy metals, in particular mercury, cadmium, chromium, copper, nickel, and zinc

•cyanides

Whether a substance is regarded as toxic or hazardous (according to the definition given above) is primarily a matter of concentration. Many heavy metals are vital as

2.3 Kinds and Impacts of Wastewater Components 55

trace elements for the growth of microorganisms, but toxic in higher concentrations.

In Germany, the regulations for hazardous substances are so extensive that certain industrial branches not only have to comply with the required discharge quality of the effluent of the entire plant, but also must meet further requirements for the locations where certain flow fractions emerge before they are mixed with the eventual effluent (fractional flow treatment). Some substances have been banned entirely, which requires a specific design and schedule of particular production steps.

2.3.8

Corrosion-inducing Substances

The evaluation of which substances are corrosion-inducing in what concentrations depends, not only on the substances themselves, but also on the choice of material.

For cement-bound materials three kinds of corrosion are distinguished: swelling impact through sulfates, dissolving impact through acids, and dissolving impact through exchange reactions, e.g., with chloride, ammonium, or magnesium. A sufficient concrete resistance is given when the following limits are not exceeded with quality concrete and long-term exposure: SO4 <600 mg L–1 (for HS concrete <3000 mg L–1; inorganic and organic acids pH >6,5; lime-dissolving carbonic acid <15 mg L–1; magnesium <1000 mg L–1; ammonium nitrogen <300 mg L–1). Biocorrosion may occur when organically highly polluted wastewater with a neutral pH becomes anaerobic. Then H2S and, in consequence, H2SO4 (which is aggressive to concrete) are produced in the gas phase.

Corrosion of metals is an electrochemical process for which a conductive liquid must be present, e.g., water. For metallic materials problems arise particularly with high concentrations of chloride, which can lead to corrosion even when high-grade steel is used. It is not possible to determine any general chloride concentrations, because the potential for corrosion depends on a number of parameters, such as the material quality, redox potential, gap width, flow velocity, temperature, and manufacturing quality.

Synthetic materials are commonly regarded as largely corrosion-proof. Some synthetics, however, are not stable toward organic or inorganic acids, alkalis, solvents, or oils, so that when dealing with aggressive media one has to consult resistance tables and check the producers’ advice.

2.3.9

Cleaning Agents, Disinfectants, and Lubricants

In industrial factories a great number of cleaning agents, disinfectants, lubricants, dyes, etc., are used. If they occur in high concentrations, many of these materials have an inhibiting or even toxic impact on biological treatment methods. It is also possible that they may contain components that cannot be eliminated in the wastewater treatment plants, e.g., AOX.

56 2 Industrial Wastewater Sources and Treatment Strategies

Although unrestricted production flow is the guiding principle for industrial companies (e.g., in food industry factories becoming unsterile must be entirely ruled out), samples tested in practice have shown that consumption control systems or replacement of particularly polluting substances have allowed for useful and costeffective improvements.

2.4

General Processes in Industrial Wastewater Treatment Concepts

2.4.1

General Information

To an increasing extent, wastewater treatment plants have changed from being pure ‘end-of-pipe’ units to being modules that are fully integrated into the production process, which is referred to as production-integrated environmental protection. The technological basis of this tendency is that production residues can often be used in other ways than disposal, that the wastewater flow often presents some ‘product at the wrong place’, and that it is simpler and more cost-effective to clean the single flow fractions individually and at higher concentrations. Thus, the general process of wastewater treatment by industrial companies can be roughly divided into production-integrated environmental protection and post-positioned wastewater purification. The borderline between these areas is not clear-cut.

The first step in any procedure of developing wastewater treatment concepts is a detailed stock-taking of the situation of the company with regard to production methods, water supply, and wastewater production. For instance, the productionspecific amounts of water, wastewater, and pollutant loads should be ascertained, as well as the characteristics of the different flow fractions of the company and the places where alkalis, acids, detergents, etc. are used.

The next step includes the gathering of various propositions for production-inte- grated measures and the examination of the different options for the post-posi- tioned wastewater purification unit. The following sections provide further in-depth information on this point.

The last step is the comparison and evaluation of the various propositions and options, the most important criteria for the evaluation being operational safety, economic viability, possibilities for sustainable production, and consideration of the overall concept (combination of industrial pretreatment and post-positioned municipal wastewater treatment plant).

2.4.2

Production-integrated Environmental Protection

The following methods should be examined with regard to their suitability as pro- duction-integrated measures. The main principle should be that avoidance should take precedence over utilization and utilization over disposal.

2.4 General Processes in Industrial Wastewater Treatment Concepts 57

•careful treatment of raw materials (short storage periods, careful handling)

•changes in the transport facilities (dry conveyance, establishment of conveyance circuits)

•changes in the production methods (reduction of water demand, substitution of water, improvements in the production organization)

•avoidance of surplus batches and production losses

•product recovery, for example, by cleaning pipes

•production circuits and multiple use of water (flume and washing water circuits, reverse-flow cleaning, lye recirculation in CIP (cleaning in place) plants, cooling water recirculation)

•utilization of raw materials from production residues (protein coagulation, valuable substance recycling, byproduct yield, forage production)

•separate collection of residues

•extensive retention of production losses in collection containers and utilization separate from the wastewater flow

•cleaning in different steps: (1) dry cleaning, e.g., with high-pressure air; (2) washing; (3) rinsing

•useful treatment of the flow fractions

•general operation and organization (training of staff, control of water and wastewater amounts, installation of water-saving devices, use of high-pressure cleaning tools, etc.)

2.4.3

Typical Treatment Sequence in a Wastewater Treatment Plant

A typical treatment sequence in a wastewater treatment plant consists of the stages listed below. Most industrial factories, however, have only a few of these operation stages, either because they do not have to cope with the respective pollutants in their wastewater or because they are exempted from particular treatment steps due to specific regulations, which occurs, e.g., with indirect dischargers.

•removal of obstructing substances (screens, grit chamber)

•solids removal (strainers, settling tank, flotation)

•storing equalization cooling

•neutralization or adjustment of the pH

•special treatment (detoxification, precipitation/flocculation, emulsion cracking, ion exchange)

•biological treatment or concentration increase (evaporation) or separation (membrane methods)

582 Industrial Wastewater Sources and Treatment Strategies

2.5

Wastewater Composition and Treatment Strategies in the Food Processing Industry

2.5.1

General Information

In view of the great variety of industrial branches, this section is by no means comprehensive in examining all areas of industry, but concentrates on the most important branches of the food processing industry.

When examining specific wastewater amounts and pollutant concentrations, it is important to consider the points already mentioned in Section 2.1 (e.g., age and technical state of the equipment used at the plants). One crucial aspect mentioned in the introduction as well is training and motivation of the employees, which can have a substantial impact on the amount of product loss (spillage, etc.) during the production process, since these losses have a considerable impact on the concentration of pollutants in the wastewater. Apart from the hard statistical facts about production processes, a conscientious plant designer should, when planning the layout of the wastewater treatment units, also try to consider all operational factors of the particular plant.

The general pressure to remain cost-effective is prodding the industry to find ever new solutions, which makes for ongoing optimization. This has a considerable impact on the amount and composition of the wastewater produced and thus directly on the treatment methods. Two basic tendencies have resulted from this: the first is the attempt to reduce the amount of water used, e.g., by recycling or reusing water in other processes. The result is a decreased amount of wastewater, but with a higher concentration of pollutants. The second tendency is to separate the fractions and to reduce the specific load (kg COD t–1 of product), e.g., by disposing of the dried solid matter (e.g., as nutrients for agricultural use). A detailed description of what is generally referred to as production-integrated environmental protection is presented in the following section.

To make precise statements about wastewater amount and composition for any particular industry, one needs to obtain the latest data pertaining to the particular plant. Apart from the relevant periodicals dealing with this topic, the most important data sources in Germany are the Handbücher zur Industrieabwasserreinigung

(manuals on industrial wastewater treatment) published by the Abwassertechnische Vereinigung ATV (Association for Wastewater Technology, Germany, today ATVDVWK) (ATV, 1999, 2000, 2001). Furthermore, relevant data can be found in the reports, codes of practice, and leaflets developed and issued by the single ATVDVWK commissions and ATV-DVWK working groups.

2.5.2

Sugar Factories

Sugar can be produced from sugarcane or sugar beets. In Europe sugar is extracted from sugar beets. The average sugar content of sugar beets is 18%. The processing