Looking for a solution to this problem, Heineken turned to Omex Environmental (OE), which provides concentrated bio-available solutions of micronutrients called Nutromex® Trace Elements Additives (TEA). It was decided to do a five month trial in UASB 2 with Nutromex TEA 310, a solution containing complexed compounds of iron, manganese, nickel, zinc, cobalt, molybdenum, copper, boron and selenium.
The anaerobic digester treats about 3000 m3/day of wastewater at a chemical oxygen demand (COD) concentration of 3200 mg/L. Following the addition of Nutromex TEA 310 in February 2010, the digester performance started to improve within one month´s use. Daily figures of several performance parameters were recorded and studied three months prior to the use of TEAs and five months afterwards.
Before dosing TEAs, the COD removal efficiency varied from 70% (even as low as 55%) to 95% whereas when using TEAs the efficiency was much more stable. In addition to this, the average COD removal efficiency increased from 81.3 % to 87%, although the COD load was about 8% higher from February onwards.
More COD removed in the digester leads to more biogas production, which actually increased by 27% on average. Apart from that, sludge activity improved with the dosing of TEAs, as the specific biogas production (amount of biogas produced per kg. of COD removed) rose by 13%. This means that the greater volume of biogas produced was due to the improved COD conversion into biogas, but there was also a higher COD load at the same time.
The amount of volatile fatty acids (VFAs) produced by the digester decreased in the TEAs trial, which could be directly related to the increased biogas production and the improved sludge activity. VFAs out of the reactor were rather unstable prior to the TEAs addition.
Cost savings
VFA concentrations could easily vary from 10 mg/L to 350 mg/l - a rather high concentration. However, after approximately one month, VFAs levels started stabilise around a concentration of 35 mg/L. The average VFA out concentration dropped about 88% - from 91.5 mg/L (before TEAs) to 48.6 mg/L. With greater methanogenesis, biogas production improved allowing a greater saving in the costs of running the brewery boilers.
The new nutrient solution also heralded an improvement in suspended solid removal, which decreased by around 16% while the sludge grew considerably - in the five months of the trial it increased from about 16,000 kg to 23,000kg.
Clearly, in general, the majority of the plant's performance parameters were positively influenced by the micronutrient mix addition while the major benefits associated with the change have helped improved digester stability and increased biogas production, yielding significant savings for Heineken Seville.