How Public Municipalities Make You Pay
The non-fermentable materials left after fermentation are just one of many challenges for ethanol plants and distilleries. For many distilleries, if the whole stillage, or subsequent products that are made from the stillage, cannot be removed from the facility in a timely fashion (this is known as “depletion” at some facilities), then processing operations may have to stop.
A rule of thumb states that for every 1 kg of cereal grain used for fermentation, approximately 1/3 kg of each of the constituent product streams (alcohol, carbon dioxide, non-fermentable residues) will be produced. On a more scientific basis, the rule of thirds arises due to the metabolism of the yeast while they consume the glucose in the grain’s starch:
1/2 C6H12O6 (Glucose) → C2H5OH (Ethanol)
+ CO2 (Carbon Dioxide)
Theoretically, 1 bushel of cereal grain can yield a maximum of about 2.97 gal/bu (0.44 L/kg). This is especially true for grain-in fermentations; grain-out fermentations, however, will often have a much lower yield of alcohol, and thus higher residual starch and a greater quantity of spent grain solids.
Realistically, however, an alcohol yield between 2.50 to 2.74 gal/bu (0.37 to 0.41 L/kg) is more common at many distilleries, although some plants can achieve up to 2.80 gal/bu (0.42 L/kg). Thus, 25.4 kg of cereal grain will typically produce about 8 kg of alcohol, 8 kg of carbon dioxide, and 8 kg of spent grains (not counting the water). This equates to a cereal grain-to-distillers grains conversion of about 0.30 kg spent grain per 1 kg cereal grain (with a range from 0.28 to 0.32 kg per 1 kg cereal grain) – thus the rule of thirds.
Potential variations in cereal grain conversion and fermentation will substantially affect the quantity of coproducts that are generated during processing. Moreover, at individual distilleries, variations in raw material inputs, equipment used, and operational procedures will also result in conversion rates that will not match values found in literature, but instead vary over both time and location.
There is much economic value to be gained from dewatering the spent grains (i.e., separating the thin stillage, which contains water and dissolved solids) and selling the distillers wet grains (i.e., suspended solids) as livestock feed. Typical nutrient compositions for thin stillage and spent wet grains are provided in Tables 1 and 2 at right.
Separation can be achieved with a variety of equipment, including centrifuges, screens, etc. Many distillers, especially small plants, do not yet do this though. If you are sending your liquid byproducts (i.e., whole stillage or thin stillage) to the city water treatment system, there are many issues that you need to consider.
Public wastewater treatment authorities have fee structures where non-residential customers (e.g., processors) are charged according to the burden that they place on a wastewater treatment facility. One component of this fee is a surcharge based on the specific chemical characteristics of your wastewater. Municipal authorities periodically sample customers’ wastewater streams and analyze them for parameters such as biochemical oxygen demand (BOD), total suspended solids (TSS), ammonia (NH3), and other chemical/physical properties. The specific chemical characteristics which are monitored vary by authority, and these parameters are then used, along with the volumetric output of wastewater, to compute each customer’s fee.
Every city in the United States is unique, however, and each has its own rate structure. But a large proportion of U.S. wastewater treatment authorities do not post their surcharge formulas on the internet, so understanding how these fees are determined is not always clear.
In a recent study (Garcia et al., 2016), data was gathered on the methods used to determine sewage surcharges for industrial customers. For each public sewage treatment authority, information on the pollution characteristics measured, rates, and methods of computation were recorded and assessed. Overall, information was collected for 71 wastewater authorities throughout the United States.
Information about the chemical characteristics used to determine surcharges from these municipal authorities is summarized in Table 3 above. Over 90% of the municipalities surveyed included both TSS and BOD in the determination of sewage surcharges. Between 21% and 31% used FOG, TKN, and COD to determine fees. Fewer than 20% of the municipalities used other chemical factors.
In the national dataset of municipalities, there were multiple ways to calculate sewage surcharges. For 60 of the 71 (84.5%), surcharge formulas were expressed with this type of formula:
Si = V × (Bi − Ci) × D × Ei
• Si is the surcharge for a given wastewater characteristic in USD. (See Table 3 for the typical characteristics.)
• V is the total volume of wastewater discharged during the billing period (gallons).
• Bi is the measured concentration of the wastewater characteristic i in the discharged wastewater (mg/L).
• Ci is the allowable concentration of the wastewater characteristic i (mg/L).
• D is a unit conversion factor (typically 8.34 lb).
• Ei is a cost factor for the wastewater characteristic (USD/lb).
If more than one wastewater characteristic is monitored by a municipality, then the total surcharge can be calculated using the following formula:
Total Surcharge (USD) = σ Si
• Si is the surcharge for a given wastewater characteristic (USD) (See Table 3.)
As shown in Table 4 above, stillage fractions (either whole stillage out of the fermentor or thin stillage after centrifugation/screening) both have relatively high BOD and COD levels.
To date, little has been published about the other wastewater characteristics of stillage. However, it is important to understand your wastewater characteristics, especially if you are planning on sending your byproducts to the city for treatment.
What’s Next?
What do all of these things mean for distilleries? If you are not selling your spent grains for livestock feed and are instead sending your byproduct streams to the city for treatment, you are spending money – the level of which depends upon your specific wastewater characteristics as well as your municipality’s fee structure.
Consider these costs as you weigh all of potential options for coproduct separation, processing, and sales. The optimal choice for you also depends upon the size of your distillery, proximity to livestock farms, transportation costs, etc. There is no one-size-fits-all approach. But as the alcohol processing industry grows, these are some of opportunities and challenges that we must all understand.
My column in the First Quarter 2025 issue of Processing Journal will examine ways to mitigate the costs of wasted byproducts.
References
Garcia, R.A., Nieman, C.M., Haylock, R.A., Rosentrater, K.A., Piazza, G.J. 2016. The effect of chicken blood and its components on wastewater characteristics and sewage surcharges. Poultry Science. 95(8):1950-1956.
Rosentrater, K. A. and Liu, K. 2011. Distillers Grains: Production, Properties, and Utilization. Boca Raton, FL: CRC Press.
Kurt A. Rosentrater, Ph.D., is a professor in the Department of Agricultural and Biosystems Engineering at Iowa State University, Ames. He can be contacted at karosent@iastate.edu or 515-294-4019.