1. Introduction
In 2018, wine production reached a record volume, with over 293 million hectoliters being produced globally [
1]. As the largest consumer, the United States is also the fourth-largest producer of wine globally [
1]. Wine produced in the United States comes from wineries of many sizes; wineries that produce over 500,000 cases per year are considered to be large, and wineries producing less than 50,000 cases per year are small [
2]. Currently, only 341 of the 10,185 wineries in the United States are classified as medium or large, and the rest are classified as small, very small, or limited production [
2]. These smaller wineries dominate the Michigan wine industry. In 2018, there were nearly 150 wineries [
3], 71% of which were ten acres or less in size [
4]. These 150 wineries bottle more than 11.3 million liters (>3 million gallons) of wine annually [
5]. It is estimated that, for every liter of wine produced, 2.86–4 L of wastewater is generated [
6,
7], which suggested that Michigan produced 45.4 million liters (12 million gallons) of winery wastewater in 2018.
The composition of winery wastewater is highly variable between wineries as a result of general management practices, the amount of water used, the size and design of the winery, and different wine-making techniques [
8,
9,
10,
11,
12]. Winery wastewater quantity and quality vary, even within a single winery due to the various steps in wine-making and the type of wine produced [
6,
9,
11,
12]. Wastewater is generated throughout the wine-making process, which has five distinct stages: harvest, crush, fermentation, racking and clarification, and aging and bottling. The harvest period produces the highest chemical oxygen demand (COD) strength wastewater and accounts for the largest volume of winery wastewater produced [
8,
13]. Small wineries may generate up to 80% of their wastewater during this period [
8]. After the harvest season, wastewater production is at a minimum and it depends on daily activities [
8].
Winery wastewater is high in organic matter from grapes and wine [
8,
9]. This organic matter contributes up to 85% of all contaminants in winery wastewater. The remaining organic matter includes yeast, alcohol, esters, sugar, soluble organic acids, tannins, lignin, and polyphenols [
9,
13].
Table 1 presents a summary of conventional wastewater pollutants in winery wastewater, as reported in the literature. In addition to organic matter, nitrogen and phosphorus are of primary concern. Proteins that are removed during stabilization of wine are the predominant source of these nutrients. The use of phosphate detergents can also drastically increase the phosphorus concentrations [
14]. Potassium and sodium are often found in high concentrations in winery wastewater due to cleaning agents and excess grape juice [
8].
Without proper treatment, the discharge of winery wastewater can result in adverse health and environmental consequences, such as methemoglobinemia in infants [
39], eutrophication, cyanobacterial blooms [
40], and cyanotoxins [
41]. Metal mobilization is an additional environmental concern when wastewater is treated while using land application. Naturally occurring metals in the soil may become chemically reduced as oxygen becomes depleted during microbially mediated oxidation-reduction reactions [
42]. Metals in the soil, such as manganese and iron, are water-soluble when chemically reduced, which allows for groundwater contamination [
42].
Regulations have been set to mitigate the impacts of wastewater discharges, and new, more restrictive regulations are driving the development of technologies for winery wastewater treatment [
6,
43,
44]. Many wineries are located rurally and they do not have access to public sewers. Those that do often face high surcharges due to the acidic pH and high COD in the wastewater [
6,
13] make onsite treatment an attractive alternative. Historically, onsite treatment of winery wastewater at small wineries has been accomplished by land application [
6], but stricter regulations have increased the land that is needed for treatment, reducing that available for vineyards. Currently, activated sludge systems represent the majority of treatment systems at European wineries [
6], but they are complex to operate for small wineries and are expensive due to high energy use [
7,
9]. Emerging treatment systems for winery wastewater include membrane bioreactors, jet-loop activated sludge, and air micro-bubble reactors, as summarized in Mosse et al. (2011) [
9]. However, these technologies may not be applicable to small-scale wineries due to their cost, complexity, and limited demonstrated applications [
9].
Further, the high variability in strength and volume of winery wastewater is challenging for small-scale wineries that treat their wastewater onsite in order to meet regulations. Northern wine regions, such as Michigan, face the additional challenge of maintaining treatment during cold winter months. For these reasons, it is necessary to develop a low cost, low complexity treatment system that requires less treatment area than traditional land application methods. The treatment system must also be able to handle the high strength and irregular production of winery wastewater, as well as maintain treatment performance during cold weather.
Subsurface vertical flow constructed wetlands (SVFCWs) have been previously used to treat diverse, high strength wastewater [
45,
46]. In a prior study in our laboratory, SVFCWs were used to biologically treat wastewater in three subsurface gravel cells, where a layer of soil above the SVFCW prevented freezing conditions [
47]. An extensive pretreatment system that included a septic tank and an effluent filter mitigated issues that were related to bed clogging [
47]. Following pretreatment, wastewater was distributed into the system at the vegetated surface during warm months or below the soil layer during cold months [
47]. All of the microbial processes occurred within the lined cells, this preventing metal mobilization. Microbial processes are the main treatment mechanism in SVFCWs, which presents some challenges. During the start-up period of the SVFCW, or after an extended period of no wastewater inflow, nitrogen might not be entirely removed, as nitrifying bacteria grow at much slower rates than heterotrophic bacteria [
48]. Additionally, it has been shown that vertical flow wetlands do not significantly remove phosphorus [
49].
Previous research on SVFCWs indicates its applicability in treating the organic matter in winery wastewater. However, additional treatment using adsorption media might be required to continuously remove nitrogen and phosphorus during cold weather and after periods of low or no flow of wastewater. Recent research has shown promising results for this type of wastewater for the sorption of ammonia while using the natural zeolite mineral, clinoptilolite [
50], and nitrate using tire chips [
51]. Sorption allows for the removal of these nutrients, while microbial communities build up and become adequate to treat the nitrogen completely; during periods of low flow of wastewater through the media, these inexpensive materials can be microbially regenerated in-place, which allows for its continued use [
51,
52,
53]. Additionally, oyster shells with a composition of 95% calcium carbonate [
54] are added to provide pH buffering [
51]. In a prior study, scrap tire chips (1–1.5 cm particle size) were found to leach small amounts of bioavailable organic carbon that supported denitrification [
51]. Although low concentrations of zinc, selenium, manganese, antimony, and cobalt were detected, other metals of concern were below the detection limits (see Krayzelova et al. [
51] for a review of toxicology studies). Moreover, tire chips have been approved for use in onsite drain field applications in several states [
55]. Engineered nano-enhanced media are becoming commercially available to adsorb phosphorus and they can be easily regenerated, and the phosphorus recovered as a fertilizer. For this research, PO4Sponge, by MetaMateria Technologies, Columbus, OH, was used. This adsorption media is a nano-enhanced iron foam that is composed of iron oxide nanocrystals of oxyhydroxide. Its alumino-silicate bonded porous structure gives it increased porosity and contact time, high adsorption rates, and an adsorption capacity ranging from 20–50 mg phosphorus/g media [
56].
This research investigates the utility of an SVFCW with nitrogen and phosphorus adsorption media to treat winery wastewater. Performance data were collected from bench-scale treatment systems under normal operating conditions, in reduced temperatures, and after periods of no-flow of wastewater.
4. Conclusions
The bench-scale SVFCW shows the potential to sufficiently treat winery wastewater under variations in loading rates, frequency, and temperature without the aid of nitrogen adsorption media or a pH buffer. Although this contradicts the original hypothesis that a SVFCW would not provide sufficient treatment without amendments, this represents a more cost-effective solution for small-scale industrial wineries. The bench-scale systems in this study demonstrated removal efficiencies for COD of >99% under constant loading frequencies and room temperature and with decreased temperatures. Removal efficiencies of 95% and 94% for total nitrogen were observed for constant loading frequencies with room temperature and decreased temperatures, respectively. The effluent concentrations were considerably better than the quality of septic system effluent, allowing for versatility in the final discharge of the treated wastewater.
This treatment system is advantageous to other conventional treatment methods due to its minimal surface area requirements, low energy demands, and high treatment performance. SVFCWs require approximately 80% less treatment area than land application, using the permitted Michigan surface loading rate of 50 pounds of BOD
5/acre/day [
80] for land application as an example. In addition to the potential economic benefit from the sale of wetland plants [
81], the use of aesthetically pleasing vegetation might be beneficial to wineries where tourism is important. Additionally, SVFCWs are substantially more energy-efficient than activated sludge [
7]. Further, the characteristics of the discharged wastewater from SVFCWs are comparable to or better than wastewater treated by conventional methods, such as activated sludge [
82]. These benefits make this technology a viable option for implementation at small-scale industrial wineries. The increased effectiveness of the addition of nitrogen adsorption media was unclear and should be further examined with a waste stream that has a more favorable C:N:P ratio. This study demonstrated that nitrogen adsorption media is not required for effective treatment of winery wastewater while using an SVFCW, but that it might enhance SVFCW treatment of wastewaters with higher nitrogen concentrations. The inclusion of the phosphorus adsorption media, PO4Sponge, was found to be an effective means of removing total phosphorus from winery wastewater to concentrations below 0.102 mg/L. Further study is needed for determining the capacity of the PO4Sponge for winery wastewater to better understand the frequency of replacing the media. However, research on other waste streams has demonstrated a media adsorption capacity of 50 mg P per gram of media with an initial phosphorus level of 7 mg/L [
83]. Additionally, other engineered adsorption media are commercially available, and waste material formulated into adsorption media, such as steel furnace slag, should be evaluated to determine which is best suited for application at a winery [
84].
A field demonstration is needed to monitor performance in a diverse environment, determine any additional design and operational considerations, and verify whether clogging will become an issue over time. A previous field study of a similarly designed SVFCW treating milking facility wastewater observed no clogging over five years of monitoring [
45]. These observations are consistent with this study, which also did not detect clogging. However, a six-month study is not indicative of the expected performance of a twenty-year treatment system [
57], and further research is needed. Additionally, this application was focused in a region where combining domestic wastewater with industrial wastewater is strongly discouraged. However, domestic wastewater could provide a year-round source of substrate to microbial communities in regions where co-treatment is allowed, justifying further investigation.