Provenance & quality
Samples processing (sampling, homogenisation, particle size reduction, storage, taking test portions, etc) was done following the recommendations of standards ISO/DIS 14820-2 and BS EN 15002:2006. The minimum amount of subsample taken from any sample type was no less than 0.5 kg.
Each digestate sample received consisted on 5-10 kg of material, which has been sourced at the end of the process line in the AD plant, placed in jerry cans or barrels, allowed to cool down and then placed in suitable containers with ice for their transport (either by researchers or courier). Once received by Lancaster University, samples were placed in a cold room (<4C) and analysed as soon as possible. Normally, a subsample of selected digestates was sent out to an accredited external laboratory for determination of complementary analyses (mainly nitrogen forms). Prior to analyses of fresh digestate samples, no further pre-processing (e.g. sieving) was carried out. For determination on dried samples, the material was dried at 60/105C until constant weight, then milled and sieved to pass a 1 mm mesh and stored in zip bags and left under room temperature until further use.
Each ash sample received consisted on 10-20 kg of material, which was sent by the producer and received by Lancaster University. From each sample type, a composite sub-sample was taken, air-dried, ground/milled sieved to pass a 1 mm mesh and placed in zip-bags left under room temperature until further use. This is what was considered as “fresh” sample. When the analytical method required fully dried samples, ashes were dried at 105C until constant weight. Normally, a subsample of selected ash was sent out to an accredited external laboratory for determination of complementary analyses (mainly phosphorus forms).
Standard methods used for the analyses of the samples. pH was determined using a pH-meter, electrical conductivity was determined using a conductivity meter; nitrate, ammonium and phosphate was determined using an automated flow analyser; water soluble (WS) and Aqua Regia soluble (AR) elemental composition was determined using inductively coupled plasma optical emission spectrometry (ICP-OES) or microwave digestion; total Nitrogen and Carbon was determined using an elemental analyser.