Growing & Winemaking

 

Capitalizing on Winemaking Waste

July 2016
 
by Cinzia da Ros, Cristina Cavinato, Paolo Pavan, Franco Cecchi and David Bolzonella
 
 
Anaerobic digestion
 

Waste flows from the winemaking supply chain contain wastewater (generally treated inside the winery or sent to municipal plants via sewage systems or by truck) and solid waste such as stems, pomace and lees. In Italy the stems were traditionally used as soil amendments, while for years the pomace and lees have been delivered compulsorily to distilleries for the production of alcohol and the recovery of tartaric acid.

Regulation EC 479\2008 of the European Union introduced important new elements to the legal framework of the wine sector, offering member states the possibility of defining the end use of pomace. Italy implemented the above regulation by means of a ministerial decree, which, as an alternative to distillation, allows it to be collected under control for the recovery of enocyanins (anthocyanins responsible for the color of red grapes), the production of agricultural products (direct or indirect agronomical use), energy recovery (using byproducts such as biomass for the production of biogas or for fueling energy-production plants) and the extraction of molecules with a high added value for pharmaceutical and cosmetic purposes.

Direct agronomical use of these waste flows is a choice to be made under careful control and is inevitably subject to the dictates of legislation related to nitrates. In fact, it should be taken into account that the presence of polyphenols (antimicrobials), salts, heavy metals and nitrogen in these highly biodegradable wastes can compromise the quality of the receiving soil in the long term. Moreover, polyphenols inhibit germination and immobilize nitrogen in the soil.

Treatment of these wastes is fundamental, but it can be a significant cost for a winery. In fact, the high chemical and biological oxygen demand values highlight the high consumption of oxygen associated with their decomposition and therefore the risk of establishing conditions of anoxia in soils and rivers.

    KEY POINTS
     

     
  • The availability of sludge and waste lees throughout the year in wineries presents an interesting resource for energy production.
     
  • Researchers in environmental science and biotechnology at the Universities of Venice and Verona (Italy) studied the potential benefits.
     
  • Their results showed that about 75% of organic matter was transformed into biogas, while nitrogen and phosphorous became more bioavailable as they became more concentrated, making the digestate a valid soil amendment.
     

Biogas from anaerobic digestion
Of the technologies that can be applied to the treatment of organic residues, anaerobic digestion (see “What is Anaerobic Digestion?”) is the most suitable process for their stabilization concomitant to the recovery of a renewable energy source such as biogas.

Production of biogas by anaerobic digestion is a mature and widely applied technology that can provide a sustainable approach from an economic and environmental point of view. Today there are more than 10,000 anaerobic digestion plants in farms all over Europe (EBA data). The anaerobic process enables us to obtain a biologically stabilized effluent with fertilizing properties that is hygienically safe.

The production of biogas convertible into heat and electricity guarantees the energy self-sufficiency of the process and provides an energy surplus that can be used to produce heat or cooling (trigeneration systems), which can be used within the cellar or in adjacent rooms.

Methanation tests: the potential of sludge and lees
To better understand the potential of anaerobic digestion applied to wine waste, the results of methanation tests with different winemaking wastes are reported (see “Methanation Test Results”). Pomace is the most interesting substratum because it has a high level of organic matter, and the production of methane (152 liters of CH4/kg) is comparable, for example, to that of corn silage. Although this waste is available for just a few months of the year, it is an interesting substitute for energy crops in autumn in plants fueled by animal farming waste.

Lees and sewage sludge, on the other hand, are available for a much longer period throughout the year. They vary depending on the size of the winery and the type of wine production; therefore, though they typically produce less biogas, their availability makes them more interesting. The lees produce methane quickly, reaching 90 liters of CH4/kg after about 20 days from the start of the process, whereas the sludge (like the stalks) has a low production of methane (26 liters CH4/kg) and needs long retention times inside the anaerobic reactor.

Considering the availability of the waste, most of the applications of the process are concentrated on the treatment of the lees and sewage sludge, leaving open the possibility of integrating the other substrata for brief periods of the year. Numerous studies (Riaño et al., 2011; Rodriguez et al.,2007; Melamane et al., 2007) have demonstrated that anaerobic digestion of different types of substrata (co-digestion) favors the stability of the process thanks to the contribution of alkalinity (buffer capacity) and nutrients to the system, thus creating a favorable environment for the growth of microorganisms without the addition of synthetic compounds.

Another advantage of co-digestion is reduction of the inhibiting or toxic effect of compounds such as polyphenols deriving from the processing of grapes, but also of heavy metals (copper and zinc) and organic micropollutants coming from sewer sludge, by effect of the dilution of these substances (Chen et al., 2008).

The experiment
In order to assess the applicability of anaerobic digestion with these winemaking wastes, an experiment was carried out using lees and sewage sludge produced by a cellar that sells more than 3.3 million cases of wine per year. The study showed that for every 100 liters of wine produced, 196 liters of wastewater has to be treated, which, in turn, determines the formation of 0.5kg of sewage sludge with 15%-20% dry matter.

The treatment of wastewater and the cost for disposal of sludge are significant expenses on the winery’s balance sheet and are difficult to recover over time. The winery in question also produces 1.6 kg of lees per hectoliter, a lower value than the national average (6 kg/hl), since it does not directly process grapes but wines bought from third-party wineries. A process of co-digestion of sewage sludge and lees has been simulated on the basis of these assumptions.

The tests were carried out in pilot reactors with 230 litres of usable volume by applying different retention times of the substratum in the reactor (23 or 40 days) and temperatures. Both the mesophilic and thermophilic conditions were tested. Reactors operating at 55° C highlighted the instability of the process and the accumulation of volatile fatty acids until the inhibition of the anaerobic process. Both of the operating processes at 37° C, on the other hand, proved to be very stable with comparable yields.

The study showed that degradability does not increase if the retention time is prolonged to 40 days. Therefore, the most interesting conditions for the full-scale application of the process are a temperature of 37° C and a retention time of about 20 days, since the dimensions of the reactor—and therefore the management costs—are limited, but the same mix is supplied.

This process worked continuously for more than 200 consecutive days without any problems arising and showed an average production of 1.2 m3 of biogas per m3 of reactor (with 65% methane).

Conclusions
More than 75% of organic matter (expressed as chemical oxygen demand) was converted into biogas. The polyphenols present in the lees were broken down, and the final concentration was lower than 50 mg/L.

The nitrogen and phosphorous present in the substratum become concentrated and more bioavailable thanks to the hydrolysis of proteins and fats during digestion, making the digestate an interesting soil amendment (see “Characteristics of Sewage Sludge, Lees and Digestate”). Moreover, the digestate is a stabilized matrix thanks to the transformation of the most-likely-to-decay compounds present in sludge and lees into biogas. Therefore, spreading the digestate does not cause bad smells and avoids the proliferation of insects in the fertilized soils.

Generally speaking, in order to sustain an anaerobic-digestion plant able to supply a co-generation group of 200kW of electricity in an average sized Italian winery without needing to purchase external biomasses, the reactor would need to be fueled with 23 cubic meters of lees daily and 53 m3 of sewage sludge with a content of at least 2% solids.

The reactor should have a volume of about 2,000 m3 and would produce roughly 2,150 m3 of biogas with a level of methane around 65%. 4,800 kWh of electricity and 6,171 kWh of heat could be obtained from combustion of the biogas. This production could cover the energy requirements of the plant itself, and the surplus could be used for other winery requirements, including the generation of energy for cooling.

Sustaining a plant of these dimensions requires a cooperative system able to focus the production of the area on just one point and distribute energy production to the production processes and neighboring rooms. An alternative to the creation of consortia systems could be to send winemaking waste to anaerobic digesters already present in the area, especially sewage plants equipped with digesters, which are generally under-loaded (Da Ros et al., 2013), or to substitute corn silage in reactors that treat animal farming waste, making the processes more sustainable.


Cinzia da Ros and Cristina Cavinato are from the Università Ca’Foscari in Venice (Italy). Paolo Pavan, Franco Cecchi and David Bolzonella are from the Università degli studi in Verona (Italy). They thank Vinicola Serena Srl for their collaboration and ATS Scarl and Treviso City Council.

Editor’s note: This article is published as part of Wines & Vines’ cooperative editorial effort with Il Corriere Vinicolo, the leading Italian wine industry publication (corrierevinicolo.com). Il Corriere Vinicolo is edited by Unione Italiana Vini, the largest Italian wine trade association. On a regular basis, our two publications will share key articles in order to give readers a broader view of important wine industry topics in Italy and North America.

 
SHARE »
Close
 
Currently no comments posted for this article.
 
 
SEE OTHER EDITIONS OF THIS COLUMN » CURRENT COLUMN ARTICLES ยป