Life cycle assessment (LCA) of cleaning-in-place processes in dairies

Abstract

Four Cleaning-In-Place (CIP) methods for dairies were compared using life cycle assessment (LCA). The methods were conventional alkaline/acid cleaning with hot water disinfection, one-phase alkaline cleaning with acid chemical disinfection, enzyme-based cleaning with acid chemical disinfection and the conventional method with disinfection by cold nitric acid at pH 2. Production of detergents, transport, the user phase in the dairy and waste management of containers were included. The user phase was found to be the most important part of the life cycle. The CIP methods with small volumes and low temperatures, such as enzyme-based cleaning and one-phase alkaline cleaning, turned out to be the best alternatives for the impact categories energy use, global warming, acidification, eutrophication and photo-oxidant formation. Milk residues flushed out in the rinsing phase were the main contributor to eutrophication, but the phosphorus and nitrogen in the detergents also influenced the results. Evaluation of toxic substances poses a methodological problem in LCA. In this study, detergents partly composed of toxic substances were included, and the overall assessment was that the one-phase alkaline cleaning method was preferable from an environmental point of view. A qualitative assessment of toxicity was performed.

Introduction

Cleaning is a key process in the dairy industry to assure safe products of high quality. However, the cleaning operations do have environmental impacts; cleaning is a water- and energy-demanding process, and cleaning agents are also used. To satisfy both the hygienic demands and the environmental requirements, as limited amounts of water, energy and cleaning agents as possible should be used. From the cleaning operations, there are also emissions due to product losses, which stick to tanks and pipes after processing. Loss of milk products to wastewater gives rise to higher figures of chemical oxygen demand (COD), biochemical oxygen demand (BOD), nitrogen and phosphorus.

In their life cycle inventory (LCI) of dairies, Høgaas Eide, and Ohlsson (1998) divided the use of energy and water, and emissions of BOD, between cleaning processes and the other dairy processes. The LCI covered the whole dairy. The contribution from cleaning was found to be of great importance. The emissions and use of resources by the cleaning processes were easily measured and registered, unlike the emissions and resource use of the other dairy processes, e.g. ventilation and cooling. According to the work of Høgaas Eide (2002), cleaning processes are major contributors to the total eutrophication potential from dairy processing: they account for 80% of the total. For energy use, the corresponding contribution was approx. 30% (Høgaas Eide, 2002).

Lorentzon, Olsson, Reimers, and Stadig (1997) studied production of coffee cream in dairies using life cycle assessment (LCA). One of the findings was that cleaning and disinfection was responsible for about half of the energy used in the coffee cream production. Nilsson and Lorentzon (1999) studied the environmental impacts of the production of milk at two dairies to find the hot spots in the dairy processing. In addition, they compared chemical disinfection with disinfection using hot water only. Production of chemicals used for cleaning and their transport were also included; these were found to be of minor importance. The energy consumption and environmental impact of the cleaning processes were not as important for the total results as had been assumed after the initial study by Lorentzon et al. (1997). No major differences in the environmental impact of chemical disinfection and hot-water disinfection were found; however, toxicity was not included due to methodological problems such as lack of data and knowledge.

Four scenarios of cleaning-in-place (CIP) methods for cleaning-in dairies, with both continuous and batch production, was assessed. These scenarios imply that rinsing water and detergent solutions are circulated through tanks, pipes and process lines without the equipment having to be dismantled. The CIP system is usually automatically controlled; CIP can be defined as circulation of cleaning liquids through machines and other equipment in a cleaning circuit (Bylund, 1995). There are also other cleaning operations in a dairy, e.g. cleaning of floors and the exterior of the equipment. In this study, only the CIP processes were examined.

The cleaning solutions are stored in a CIP station, i.e. tanks connected to the dairy equipment as illustrated in Fig. 1 (Bratland & Rosenberg, 1992). The most commonly used CIP method in Norwegian dairies is the ‘traditional’ conventional one. First, as much as possible of the milk residues (border milk) are collected in a tank by means of water flushing. This milk is used as fodder. When the milk is diluted below a given level, rinsing takes place; the milk residues are flushed out. Milk has a eutrophication potential; 1 L of milk corresponds to 224 g COD, 6 g nitrogen and 0.95 g phosphorus (Selmer-Olsen, 1992).

Alkaline cleaning is the next step, and it is performed in every cleaning operation. According to Strand (1989), alkaline detergents dissolve fat and proteins (Strand, 1989). The frequency of acidic cleaning varies: every day, once a week or every other day acidic cleaning takes place. The acidic detergents remove mineral deposits (milk stone) in the equipment (Strand, 1989). Especially warm areas, e.g. in the pasteurizer, are sensitive to mineral deposits. Management and experience differ among operators and dairies as regards acidic cleaning. Disinfection by hot water or a chemical disinfectant is the final step of the cleaning operation. After chemical disinfection, rinsing with water is prescribed. According to British Standard, BS 5283:1986, disinfection is defined as the destruction of microorganisms but usually not of bacteria spores (BSI, 1986). Disinfection does not mean that all the microorganisms are killed: rather, there is a reduction of the number to a level at which neither the quality of food with limited shelf-life nor human health are endangered (Sundheim, 1999).

The CIP methods investigated in the case study were:

• Conventional alkaline/acidic cleaning by nitric acid and sodium hydroxide followed by hot-water disinfection (acidic cleaning is included every second time);

• one-phase alkaline cleaning with acid chemical disinfection;

• enzyme-based cleaning with acid chemical disinfection; and

• conventional alkaline/acidic cleaning with disinfection by cold nitric acid at pH 2 (acidic cleaning is performed every second time).

The type and concentration of each ingredient used in the detergents for the CIP methods are given in Table 1.

New detergents and CIP methods have been introduced into the dairy market in the past few years. Although some of the methods promise less energy and detergent consumption, they may give rise to emissions of environmentally hazardous substances. Surfactants or tensides, which are important compounds in complex detergents, reduce the surface tension (Strand, 1989). Stalmans et al. (1995) have made inventories of a large number of detergent surfactants by means of the LCA methodology; the methodology of this study was also described in Janzen (1995). The production of the surfactants, from the extraction of raw materials to transport and production, was taken into account. Weidema, Pedersen, and Drivsholm (1995) have pointed out that some components of detergents may affect human or ecosystem health. According to Mattsson (1999), the energy use for manufacture of cleaning agents in the production of carrot purée was negligible. Since applicable characterization methods for toxicity were not available, a red-flag classification was made to identify the most relevant substances.

In an earlier report, Høgaas Eide et al. (1998) studied the environmental impact of some CIP methods used in Norway, on which this paper is based. Cognis Industrial Consulting GmbH (1998) compared conventional cleaning, one-phase cleaning and an enzyme-based CIP-method in the cold milk area of a German dairy. The enzyme-based CIP method was found to consume the least energy and water; it also has the overall lowest environmental impact. However, toxicity or any impacts eventually caused by the enzymes were not assessed.

The main objective of this work was:

• to compare the environmental impact of new and commonly used CIP methods, simulated in a model dairy, including production of detergents, transport to the dairy, the user phase and waste management of the packaging.

Other objectives were:

• to study the importance of the transportation and the influence of different waste-water treatment methods, and to identify the hot spots in the systems studied;

• to determine the share of nitrogen, phosphorus and COD contributing to eutrophication from detergents and products originating from cleaning operations; and

• to study the feasibility of using LCA in production systems where toxic substances play an important role.

Detergents used for CIP methods contain several chemicals, which have various effects on the environment and on the human health. The assessment of toxic substances in the methodology of LCA is still under development. According to Guinée (1995), toxicity is a problem category because of the lack of data and knowledge. Data on the inherent properties of the substances, such as vapour pressure and degradation rates for soil, water and air, are needed for each chemical and for its metabolites. These data are often lacking because very few studies have been made. Understanding of how toxic substances are dispersed in the environment and the effect they have on different types of organisms is also limited. Work is in progress to remedy this problem and to incorporate toxicity characterization methods in the LCA methodology. Some of the models are listed below.

The uniform system for the evaluation of substances (USES) is a tool for quantitative assessment of the general risks of substances, designed for use in risk assessment but adapted to use in LCA (Guinée et al., 1996). The Environmental Design of Industrial Products (EDIP) is a quantitative method for the assessment of how emissions affect both ecotoxicity and human health (Hauschild, Wenzel, Damborg, & Tørsløv, 1998). The European Union risk ranking method (EURAM) calculates two scores: one for environment and one for human health (Eriksson, 1999). The CPM (Competence Center in Environmental Assessment of Product and Material Systems) method assesses the ecotoxicity of both organic and inorganic substances. First, a qualitative assessment is made to reduce the number of substances. Second, impact factors are calculated (Molander & Thurén, 1993). Scheringer, Halder, and Hungerbühler (1999) presented a model to assess the ecotoxicity of emissions to water. The Jolliet and Crettaz CST-95 (Critical Surface Time) method of assessment of human health and ecotoxicity can be used for both organic and inorganic substances. Values have been calculated for a limited number of substances (Jolliet & Crettaz, 1997). Common to the models is the requirement for extensive data on each substance, such as bio-concentration factor (BCF), octanol–water partition coefficient, the degree of biodegradation, water solubility, vapour pressure and Henry's law constant. In addition, toxicological values for various species are required. The greatest problem is to find the data required, since only a few substances have been examined and documented well enough. Eriksson (1999) compared four models for ecotoxicity of tyre emissions, but could include only some of the substances because of the lack of data.

Hertwich, Pease, and McKone (1998) compared four methods for the assessment of human toxicity: toxicity-based scoring (TBS), a sustainable process index (SPI), concentration/toxicity equivalency (CTE) and human toxicity potential (HTP). The first method, TBS, assesses only the inherent toxicity of a substance. The second, SPI, takes the exposure potential (persistence) of a substance into account in addition to the toxicity. The CTE method includes a fate model, while HTP, a method of Guinèe and Heijungs, has a multipathway exposure model, in addition to the other health-risk assessment components. Keller, Wahnschaffe, Rosner, and Mangelsdorf (1998) identified some weaknesses in the existing methods for the assessment of human health, e.g. that the approaches are not appropriate for all kind of substances. A semi-quantitative screening method was suggested. First, the substances were classified into four potentially toxic classes, i.e. based on risk-phrases (R-phrases). Second, a qualitative exposure assessment was made.

For quantitative methods that cover both ecotoxicity and toxicity to humans, a large amount of data is required on the inherent properties of substances, as well as knowledge of how they are dispersed in the environment, and how they affect different types of organisms. For most substances, such data are not available, which is also true for the compounds of the cleaning agents included in this study. In addition, one nonionic active tenside was not identified, but its R-phrases and corresponding risk class was found. Based on this, a qualitative assessment of the toxic substances based on R-phrases and risk classes (KIFS, 1994), as well as the qualitative evaluation by Svärd and Wahlberg (1998), was performed. In KIFS (1994), chemical products are classified and marked in accordance with directive 1999/45/EC relating to the classification, packaging and labelling of dangerous preparations (European Parliament, 1999). In Svärd and Wahlberg (1998), different compounds of detergents are qualitatively evaluated with regard to environmental and health effects based on both R-phrases and available data of each substance. If problems with identification of substances arise, the R-phrases from the product data sheet still could be used in a qualitative assessment. However, a quantitative assessment is not possible, until the identity of the substance is known. After qualitatively classifying the substances, the amounts of them emitted, calculated as the worst case, were compared.

Several new CIP methods and detergents for cleaning-in dairies, assumed to influence the environment less, have now been introduced. However, the toxicity of the detergents should also be assessed to find the preferable CIP method from an environmental point of view.