ELSEVIER Bioresource Technology 48 (1994) 21-24 © 1994 Elsevier Science Limited Printed in Great Britain. All fights reserved 0960-8524/94/$7.00 NITRITE PLUS NITRATE RECOVERIES IN PIGGERY SLURRY BY DIRECT DISTILLATION AND MODWIED KJELDAHL METHODS S. J. Dimmock Silsoe Research Institute, Wrest Park, Silsoe, Bedford, UK, MK45 4HS & J. Martinez CEMA GREF, Division Production et Economie Agricoles, 17 avenue de Cucill~, 35044, Rennes, France (Received 3 August 1993; revised version received 8 October 1993; accepted 15 October 1993) Abstract Aerobically treated slurry may contain nitrite (NO2-N), and nitrate (NO3-N) which are not detected by standard Kjeldahl total nitrogen determinations. The object of this work was to attempt a recovery of total nitrogen, using fresh, untreated slurry dosed with standard sodium nitrite and nitrate solutions. Three pretreatment proce- dures were undertaken to include NO2-N and NO3-N, by modification of the Kjeldahl method as follows: (a) Devarda's alloy; (b) reduced iron (Fe); (c) potassium permanganate plus reduced iron (KMnO 4 + Fe). Each of these was followed by a standard macro-Kjeldahl diges- tion and distillation. Pretreatment methods (a) and (b) gave reasonable recoveries of NO3-N added to slurry (82% and 89%, respectively), but the effect when compared to method (c) (99% recovery), was not significant. For NO2-N added to slurry there was a significant difference between methods (a) and (b) (49% and 39% recovery), and method (c) (98% recovery). Method (c) is recommended as a global method to cope with the presence of NO2-N or NO~-N in total nitrogen determinations on slurry. Inorganic nitrogen forms (NH4+-N, NO2-N and NO 3- N) were measured on the same fresh slurry by a direct distillation recovery, using magnesium oxide plus Devarda's alloy. Recoveries for added forms of nitrogen by this method were greater than 91%. Key words: Slurry, total nitrogen, Kjeldahl nitrogen, inorganic nitrogen. INTRODUCTION Raw pig slurries usually contain nitrogen in its reduced form, mainly as ammonium nitrogen (NH4 +- N), and organic nitrogen. Aeration strategies are often recommended to reduce the environmental impact of animal wastes. Such treatments can assist in odour control and also may remove some nitrogen either by 21 ammonia volatilisation or through nitrification of NHg+-N, which produces inorganic nitrogen forms such as NOE-N and NOa-N , followed by a denitrifica- tion process (Sneath et al., 1992). It is therefore important in monitoring aeration processes that total nitrogen measurements include these oxidised forms. Research has shown that Kjeldahl nitrogen tech- niques performed in routine soil and plant analysis do not accurately determine the levels of nitrate-nitrogen present (Guiraud & Fardeau, 1977). Predigestion procedures (including a priori nitrate reduction) have proved effective on soil and plant material (Olsen, 1929; Guiraud & Fardeau, 1977; Pruden et al., 1985; Goyal & Hafez, 1990). Mahimairaja etal. (1990)deve- loped a similar approach for fresh and composted manures. The present work examined the effectiveness of some of these procedures, which included a pretreat- ment step for NO2-N and NO3-N, on pig slurry samples. The approach was to determine the recovery of nitrogen from standard solutions of sodium nitrite (NaNO2) and sodium nitrate (NaNO3) , and from mixtures of standards plus raw pig slurry. METHODS Standard solutions of NO2-N and NO3-N (1 g litre-t) were prepared from dried sodium nitrite and sodium nitrate, respectively. In the trials, 5 cm3 of each solution was analysed separately, and with fresh untreated pig slurry (obtained locally) with a total solids content of 6.5%. The amount of slurry used is indicated in the tables. A semi-quantitative determina- tion for NO2-N and NO3-N was undertaken using Merckoquant test strips, to give an indication of any unexpected nitrogen forms in the untreated slurry. This proved negative in every case, and allowed the measurement of inorganic nitrogen by direct distilla- tion (as NH4+-N) (Bremner & Keeney, 1966). The