**2.4 Phosphorus**

316 Food Industrial Processes – Methods and Equipment

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

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

NH3+ H2O ↔ NH4

colourimetrically, titrimetrically or with specific ion electrodes (APHA, 1998).

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

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

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

++ OH- (1)

because the organic matter content is usually quite high.

the solution according to the equilibrium reaction:

wastewaters is from 15 to 20 mg L-1 (Metcalf & Eddy, 2003).

(Metcalf & Eddy, 2003).

**2.3 Nitrogen** 

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

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 determined as an orthophosphate (APHA, 1998).

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 significant than phosphorus (Hiisvirta, 1976).
