Determinación coliformes fecales, Método de sustrato cromogénico

The dairy industry plays an economically important role in the agricultural sector; CW, that represents approximately 90% of the employed milk (from a massive point of view), is challenging to dispose, in particular for Small to

Medium Enterprises (SMEs), because typically they do not possess the economic resources required for a proper treatment and valorisation [86]. In fact, these companies usually prefer to give away this residue for farm animal feeding (this is actually done also by the analysed dairies, located in Friuli-Venezia Giulia mountain area), and, sometimes, untreated CW is directly discharged into the municipal sewage system, causing serious environmental hazards, as well as significant problems to municipal WWTPs [81].

Anaerobic digestion (AD) can be a triple action process for CW treatment: pollution discharge reduction, energy obtainment, and nutrient recovery [87]. The successful application of AD to CW depends on the physicochemical composition of CW, in terms of organic matter, reduced alkalinity, rapid acidification tendency, as well as on the inoculum source (that needs to provide high buffer capacity) and reactor configuration [188].

In fact, inhibition by acidification is a common problem encountered during AD of acidic substrates, such as CW. This was actually experienced also in this work, in particular in BMP tests, executed at I/S=2 (Chapter 4), where methane production stopped just after a few digestion days. However, by increasing I/S ratios, high CH4production was obtained from selected CW samples. For SMEs, in literature it was suggested to use low-cost tubular digesters, that improve process stability, through separation of acidogenic and methanogenic phases; this solution can be particularly interesting for little facilities, that are not typically able to sustain high investment costs [189].

AD is known for its effect on organic matter stabilisation and removal; how- ever, tipically most of the nutrients remain in the digestate, that is characterized by N/P ratios between 2 and 4 [190]. Although this digestate has good fertilizing properties, its direct application to crops has disadvantages, such as ammonium emissions during irrigation [191] and introduction of pathogens to the fields [192]. To solve this issue, in recent years practical solutions have been proposed, to recover nutrients from the digestate, such as struvite (magnesium ammonium phosphate hexahydrate, MgNH4PO4.6H2O) [193]. Struvite is formed as crystals (fig. 6.1), that naturally precipitate when the molar ratio Mg:NH4:PO4 is above 1:1:1 [194], and is characterized by a lower water solubility, in comparison with commercial fertilisers, improving its yield and inhibiting the uncontrolled dispersion of nutrients in the environment [195]. It should be observed that only 20% of the N consumed by cows is present in milk and meat, while the other 80% is disposed of as manure and urine; so, an inappropriate digestate application allows NH3and NOx emissions.

Struvite, instead, is considered a high quality fertilizer, a fire retardant, and an absorbent for removing pollutants from the soil [197]. Transformation of

Figure 6.1: Struvite granules, recovered from liquid hog manure [198]

digestate nutrients into struvite is an environmentally friendly and sustainable method, that can remove residual pollutants, and yield profits for waste treatment in rural and mountain areas.

As for UASB process, that was the main focus of this research, the high concentration of phosphates found in raw cheese whey (Chapter 3), and the limited nutrients removal, that was typically observed in UASB processes, boost for nutrients recovery from the effluent, even if it must be underlined that Mg content in dairy effluents is not sufficient, and must be integrated. In general, phosphate content (high concentrations of PO43-, > 500 mg/L, were found in the analysed whey) determines the maximum amount of struvite that can be obtained after precipitation [86].

It is therefore possible to recover both energy (through biogas) and nutrients (through struvite precipitation) from CW. As for general technical considerations, it must be considered that CW production varies during the year, because of climatological conditions, and CW storage (that can be performed in tanks, such as the one shown in fig. 6.2) could be a solution, to compensate for the lack of substrate during the dry season. CW storage involves a decrease in organic matter content and pH, but it has been shown that this does not significantly influence methane production [196].

Given the fact that all the analysed dairies were SMEs, however, it appears advantageous to choose simple technologies for valorising this substrate, rather than UASB processes, that could be tricky to design and operate: choosing

Figure 6.2: CW storage tank [199]

a simple configuration, such as low-maintenance tubular digesters, each dairy could operate its own reactor, and the obtained biogas could be used locally, reducing energy consumption.

In addition, a synergistic effect would be obtained if AD technology ben- eficiaries were the milk producing farms, that furnish raw milk to the dairy companies, because cattle farms have access to manure, that can be used as inoculum for AD reactors start-up, and CW could be transported by unifying milk collection route from the farms with the transport of whey back from the dairy companies. Payback period for the installation of a simple plastic tubular digester, coupled with struvite precipitation, was calculated as one year in [189], so the feasibility of whey AD was confirmed also at little scale, and a synergism between SMEs and dairy farms should be encouraged, for energy production and nutrients recovery purposes.