**2.1 Total suspended solids**

The most important physical factor of wastewater is its total solid content which is comprised of floating, settleable and colloidal matter, and matter in a solution. In the characterisation of various solids in wastewater, samples can be classified in ten different fractions. The most important fraction of these is total suspended solids (TSS) which is one of the two universally used effluent standards (along with BOD) to follow the performance of wastewater treatment plants (Metcalf & Eddy, 2003). The TSS of wastewater is determined according to APHA standard 2540 D. "Total Suspended Solids Dried at 103-105 °C" (APHA, 1998).

The solid matter of food industry wastewater can also vary considerably. For example in the slaughterhouse and meat industry wastewaters, the solid matter is composed of hairs, feathers, bowels and piece of tissues (Hiisvirta, 1976) compared to potato and vegetable industry wastewaters whose suspended solids are soil, peels and other vegetable parts (Lehto et al., 2007). Typically, food industry wastewaters contain lots of floating suspended solids which have to be removed since releasing it directly to the watercourse increases sediment (Metcalf & Eddy, 2003).

#### **2.2 Organic matter**

Organic compounds consist mainly of carbon, hydrogen and oxygen. The organic matter in wastewaters is typically a mixture of proteins and carbohydrates as well as oils and fats. In the slaughterhouse and meat industry wastewaters, for example, the organic content is mainly composed of grease which can be solid, suspended or emulsified (Hiisvirta, 1976). The low solubility of fats and oils reduces the rate of their biological decomposition and in the wastewater treatment plant, fats can block up the treatment devices of wastewaters. If grease is not removed before the discharge of treated wastewater, it can interfere with the biological life in surface waters. Wastewater also contains urea and small quantities of very large number of simple and extremely complex synthetic organic molecules (Metcalf & Eddy, 2003).

There are a number of different analyses to determine the organic content of wastewater. The analyses can be divided into those used to measure aggregate organic matter content of wastewater and those analyses used to quantify individual organic compounds (APHA, 1998). Typically, only aggregate organic matter content is measured. Those are: biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total organic carbon (TOC) (Metcalf & Eddy, 2003).

#### **2.2.1 Biochemical oxygen demand**

314 Food Industrial Processes – Methods and Equipment

Pulp and paper industry 677058 14787 161 2347 10148 160813 Metal industry 208613 493 2 224 4 70 Chemical industry 115192 1193 14 435 217 2174 Mining and quarry industry 20449 431 1 69 7 686 Food industry 4067 83 4 87 99 290 Others 104981 628 9 42 272 1819 **Total 1130360 17615 191 3204 10747 165852**  Table 1. Industrial discharge into the watercourse in 2008 in Finland (Finnish Environment

The most important physical factor of wastewater is its total solid content which is comprised of floating, settleable and colloidal matter, and matter in a solution. In the characterisation of various solids in wastewater, samples can be classified in ten different fractions. The most important fraction of these is total suspended solids (TSS) which is one of the two universally used effluent standards (along with BOD) to follow the performance of wastewater treatment plants (Metcalf & Eddy, 2003). The TSS of wastewater is determined according to APHA standard 2540 D. "Total Suspended Solids Dried at 103-105

The solid matter of food industry wastewater can also vary considerably. For example in the slaughterhouse and meat industry wastewaters, the solid matter is composed of hairs, feathers, bowels and piece of tissues (Hiisvirta, 1976) compared to potato and vegetable industry wastewaters whose suspended solids are soil, peels and other vegetable parts (Lehto et al., 2007). Typically, food industry wastewaters contain lots of floating suspended solids which have to be removed since releasing it directly to the watercourse increases

Organic compounds consist mainly of carbon, hydrogen and oxygen. The organic matter in wastewaters is typically a mixture of proteins and carbohydrates as well as oils and fats. In the slaughterhouse and meat industry wastewaters, for example, the organic content is mainly composed of grease which can be solid, suspended or emulsified (Hiisvirta, 1976). The low solubility of fats and oils reduces the rate of their biological decomposition and in the wastewater treatment plant, fats can block up the treatment devices of wastewaters. If grease is not removed before the discharge of treated wastewater, it can interfere with the biological life in surface waters. Wastewater also contains urea and small quantities of very large number of simple and extremely complex synthetic organic molecules (Metcalf &

There are a number of different analyses to determine the organic content of wastewater. The analyses can be divided into those used to measure aggregate organic matter content of wastewater and those analyses used to quantify individual organic compounds (APHA, 1998). Typically, only aggregate organic matter content is measured. Those are: biochemical

**Wastewater TSS Ptot**

**t/a**

**Ntot BOD7 CODCr**

**Industry 1000 m3**

Institute, 2009).

°C" (APHA, 1998).

**2.2 Organic matter** 

Eddy, 2003).

**2.1 Total suspended solids** 

sediment (Metcalf & Eddy, 2003).

The most widely used parameter for the determination of organic matter in wastewater is BOD. In this method, the biodegradable organic matter of wastewater is measured. The biochemical oxygen demand is the amount of oxygen which organic matter (solid or dissolved) in the water is consumed when biodegradation occurs in biological oxygen containing states (SFS-EN 1899-1). There are numerous standards for the determination of BOD such as SFS-EN 1899-1 (SFS-EN 1899-1), APHA standard 5210 B (APHA, 1998) and OECD 301 F-guide (OECD, 1992). For wastewater samples, the standard measuring time is five days (BOD5) at 20 °C, but other lengths of time and temperatures can also be used. In Finland, for example, the typical measuring time is seven days (BOD7) (Karttunen, 2003).

Although there is a high content of organic matter in food industry wastewaters, the organic compounds, such as fats and proteins, are usually easily biodegradable. Furthermore, the large amount of micro-organisms, for example in the slaughterhouse and meat processing industry wastewaters, facilitates the decomposition of organic compounds (Hiisvirta, 1976). However, there are also some exceptions such as food industry wastewaters containing salts, disinfectants and cleaning agents (Finnish Food and Drink Industries` Federation, 2005).

#### **2.2.2 Chemical oxygen demand**

Chemical oxygen demand describes the number of chemically oxidising organic compounds of wastewater (SFS 3020). The COD value can be determined according to APHA standard 5220 "Chemical oxygen demand (COD)". For wastewaters, the oxidising agent is dichromate in an acid solution (APHA, 1998). The ratio of the BOD and COD can provide more information about the wastewater sample. Usually, for industrial wastewaters, COD is higher than BOD because many organic substances which are difficult to oxidise biologically can be oxidised chemically. If the COD value is much bigger than the BOD value, the organic compounds in wastewater are slowly biodegradable (Hiisvirta, 1976). In food industry wastewaters, the COD and BOD values are often closely matched to each other due to the easily biodegradable organic compounds of the effluent (Finnish Food and Drink Industries` Federation, 2005).

#### **2.2.3 Total organic carbon**

Total organic carbon (TOC) describes the amount of organic compounds in wastewater and is used as a more convenient and directs expression of the total organic content than either BOD or COD. TOC analysis provides different information to BOD or COD because the unit of the TOC value is [mg C L-1] while measuring BOD and COD uses the unit of mg O2 L-1. TOC is also independent of the oxidation state of the organic matter and does not measure other organically bound elements, such as nitrogen and hydrogen, and inorganic compounds that can contribute to the oxygen demand measured by BOD and COD (APHA, 1998). The analysing methods for TOC utilises heat and oxygen, UV radiation, chemical oxidants, or a combination of these to decompose the organic compounds of the sample to

In food industry wastewaters, the amount of nitrogen is typically bigger than the amount of phosphorus whilst the total nitrogen content can be even ten folds compared with municipal wastewater (Finnish Food and Drink Industries` Federation, 2005). In slaughterhouse and meat industry wastewaters, for example the decomposition of proteins

311

Phosphorus is also an essential nutrient to the growth of biological organisms. Due to noxious algal blooms occurring in surface waters, domestic and industrial waste discharges

The usual forms of phosphorus found in aqueous solutions include orthophosphate (e.g. PO43-, HPO32-, H2PO4-, and H3PO4), condensed phosphates (pyro-, meta-, and other polyphosphates) and organic phosphate. Phosphorus analyses include the conversion of a phosphorus form to dissolved orthophosphate and the colourimetric determination of this dissolved orthophosphate. Orthophosphate can be determined directly by adding an ammonium molybdate (forming a coloured complex), while condensed and organic phosphates must first be converted to orthophosphates by digestion before being

In food industry wastewaters, phosphorus occurs as an organic phosphate that originates from proteins and some detergents used by machine washing which may contain phosphorus. However, the nitrogen content of food industry wastewaters is more

Unpleasant odours in food industry wastewater are usually caused by gases produced by anaerobic decomposition of organic matter. The most common odour causing compound is hydrogen sulphide whose characteristic odour is that of rotten eggs (Metcalf & Eddy,

Sulphur is required in the synthesis of proteins and is released in to the degradation process. Under anaerobic conditions, sulphate is reduced biologically to sulphide which can further combine with hydrogen forming hydrogen sulphide (H2S). This gas is readily soluble in water, colourless and inflammable, but also toxic. Although hydrogen sulphide is the most common gas formed during the anaerobic decomposition of organic matter when considering odours, other volatile compounds, such as indole, skatole and mercaptans, may

In recent years, the control of odours has become more important in the designing and operating of wastewater collection, treatment and disposal plants. Odours are the foremost concern of the public in wastewater treatment processes. Quite often, the psychological stress causing by odours is far more important rather than the harm they do to the health of

Unpleasant odours are detected by the olfactory system, but which the precise mechanism is not well known. One of the difficulties in developing a global theory has been the insufficient explanation of why compounds with different molecular structures may have similar odours. Nowadays, some agreement has been achieved that the odour of a molecule has to be related to the molecule as a whole. Malodorous compounds in untreated

raises the amount of nitrogen in the effluent (Hiisvirta, 1976).

determined as an orthophosphate (APHA, 1998).

significant than phosphorus (Hiisvirta, 1976).

may contain 1 – 2 mg L-1 of phosphorus (P) (Metcalf & Eddy, 2003).

cause odours far more unpleasant than H2S (Metcalf & Eddy, 2003).

**2.4 Phosphorus** 

**2.5 Odours** 

humans (Droste, 1997).

2003).

carbon dioxide which is measured with an infrared analyser, conductivity or by some other method (Metcalf & Eddy, 2003). The inorganic carbon content of the wastewater sample can be many times greater than the TOC fraction. Therefore, the inorganic carbon (CO2, carbonates) must first be eliminated by acidifying samples to pH 2 or less to convert inorganic carbon species to CO2. Alternatively, the inorganic carbon interference may be compensated for by separately measuring total carbon (TC) and inorganic carbon (IC). The TOC can be calculated from the difference between TC and IC. There are different methods available for the determination of TOC such as APHA standard 5310 (APHA, 1998) and SFS-EN 1484 (SFS-EN 1484, 1997). Nowadays, the TOC analysis is more favourable since its measuring time is quite short (5 to 30 minutes) compared to BOD determination which takes several days before the results are known (Metcalf & Eddy, 2003). For food industry wastewaters, TOC measurement provides practical information about the water sample because the organic matter content is usually quite high.

#### **2.3 Nitrogen**

Nitrogen is an important nutrient for microbes and other biological organisms. The chemistry of nitrogen is complex, because of the existence of several oxidation states in the element. The most common and important forms of nitrogen in wastewater are ammonia (NH3), ammonium (NH4+), nitrogen gas (N2), nitrite ion (NO2 -) and nitrate ion (NO3-). Overall, total nitrogen Ntot in wastewater is composed of organic nitrogen, ammonia, nitrite and nitrate. Organic nitrogen is determined using the Kjeldahl method (APHA, 1998) where the aqueous sample is first boiled to remove any ammonia, and then wet combusted. During wet combustion, the organic nitrogen is converted to ammonium. In aqueous solution, ammonia nitrogen exists as either ammonia gas or ammonium ion, depending on the pH of the solution according to the equilibrium reaction:

$$\text{NH}\_3\text{+H}\_2\text{O} \leftrightarrow \text{NH}\_4^+ + \text{OH}^\cdot \tag{1}$$

At pH 7, over 98% of the ammonia nitrogen is ammonium ion and when the pH is increased, the equilibrium is displaced to the left. Ammonia is determined by raising the pH, distilling off the ammonia with the steam produced during sample boiling, and condensing the steam which absorbs the gaseous ammonia. The measurement can be made colourimetrically, titrimetrically or with specific ion electrodes (APHA, 1998).

Nitrite nitrogen is determined colourimetrically (APHA, 1998). It is relatively unstable and is easily oxidised to the nitrate. The amount of nitrite in wastewaters is seldom above 1 mg L-1. Although present in low concentration, it is important to determine the amount of nitrite because of its extreme toxicity to most fish and other aquatic species (Metcalf & Eddy, 2003). Nitrate nitrogen, which can also be determined colourimetrically (APHA, 1998), is the most oxidised form of nitrogen found in wastewaters. The typical range (as nitrogen) detected in wastewaters is from 15 to 20 mg L-1 (Metcalf & Eddy, 2003).

Ammonium nitrogen, also found in wastewater, can oxidise microbiologically in to a nitrate form (nitrification) and consumes vital oxygen in water systems. The nitrification consumes a relative high amount of oxygen: 1 g ammonium nitrogen needs 4.3 g oxygen for the oxidation process, a reason why ammonium nitrogen has to be converted to nitrate and for the removal of total nitrogen from wastewaters before discharging in to the watercourse (Metcalf & Eddy, 2003).

In food industry wastewaters, the amount of nitrogen is typically bigger than the amount of phosphorus whilst the total nitrogen content can be even ten folds compared with municipal wastewater (Finnish Food and Drink Industries` Federation, 2005). In slaughterhouse and meat industry wastewaters, for example the decomposition of proteins raises the amount of nitrogen in the effluent (Hiisvirta, 1976).
