What is ZLD?
Generally speaking, zero liquid discharge refers to a process that maximizes water recovery from a wastewater source otherwise destined for disposal. Salts and other solids are produced from wastewater and are usually disposed of in landfills. When all wastewater is purified and recycled, zero liquid is discharged at the end of the treatment cycle of zero-liquid discharge (ZLD).
Economic growth results in massive energy consumption, which leads to a series of environmental issues worldwide. Governments have been making strict emission standards for environmental protection. In the Paris Agreement, a universal environmental target to keep global warming below 2 degrees Celsius. This led to ambitious decarbonization goals set for most developed countries to support with policies and laws. COP26 reaffirms the temperature goal in the Paris Agreement and phased out low-efficiency fossil fuel subsidies. The UK parliament passed an amendment for cutting emissions in 2019 to achieve the pollution reduction ambitions, which set a zero discharge emission precedent.
Ethanol production: focussing sustainability
The process that Khaitan implemented not only produces ethanol but also converts waste streams into high-value-added by-products by undergoing multistage refinery steps. Inorder to achieve zero liquid discharge, additional waste processing methods are carried out. It is possible to avoid water pollution in the proposed process due to an extra water recycling step.The use of fossil fuels as operation energy, however, results in increased CO2 emissions and reduced water pollution. To ensure the proposed bioethanol plant has an environmental advantage over traditional ethanol refinery plants, the CO2 emission per kWh for all kinds of electricity should not be over 0.11 kg/kWh. Thus, the proposed concept of zero discharge bioethanol plants could establish in Countries with access to sufficient renewable electricity supply.
Lignocellulosic biomass is one promising renewable resource due to its low price, abundance and efficient conversion technologies. On the one hand, the technologies to convert lignocellulosic biomass into biochemicals, such as biodegradable plastics, succinic acid and ethanol, are mature. These chemicals have the potential to replace fossil fuels-derived chemicals by providing technological support. Besides, the feedstock supply could ensure due to the abundance of availability globally.
Components of Bio refineries
In order to achieve zero liquid discharge, biomass to ethanol biorefinery plant includes the following nine main process steps. (1) feed handling. (2) feedstock pre-treatment. (3) enzyme production. (4) hydrolysis and fermentation. (5) distillation. (6) combined heat and power generation. (7) wastewater treatment. (8) storage. (9) utility management (water system and power system),
At first, the feedstock undergoes loading and shredding for downsizing. Then pre-treated at a high temperature to decompose lignocellulosic biomass into its components like lignin and cellulose. For high-efficiency hydrolysis. Sulfuric acid, a proven competitive low-cost and high-efficiency pre-treatment solution widely applies in feedstock pre-treatment. Then, the pretreated feedstock mixes with an enzyme. This is produced in the enzyme production process for hydrolysis and fermentation under a suitable reaction condition. Finally, the glucose and pentoses hydrolyzed from cellulose and hemicellulose undergo catalyzing by an enzyme converting to ethanol. The ethanol distillation process will separate ethanol, lignin and stillage. The ethanol and stillage further process for storage and wastewater processing (the grey flow chart in, respectively. The storage plays a crucial role for elemental sources supplying to bioethanol plants.
Extracted lignin from ethanol distillation and biogas from zero liquid discharge undergo combustion to produce energy mainly heat, power and steam. This is for biorefinery plant operation and electricity grid to increase plant profitability. The ash disposal is used for agricultural purposes in such cases. The utilities include on-site recirculation of cooling water and external electricity from the grid to support biorefinery plant operation.
It is clear that various waste streams such as CO2 and wastewater discarded to the environment causes secondary pollutants, simultaneously reduce the benefits to sustainable development. However, its original intention was to reuse agricultural waste and protect the environment. For instance, in ethanol refinery process, stillage from ethanol distillation contains abundant organics. This high-value contents convert to low-value biogas for combustion. Lignin is a substantial potential raw material for the chemistry industry. In comparison, most of the lignin in traditional bioethanol refineries will burn for power generation, which causes not only source waste but also environmental impact. There is no doubt that the lignocellulose-based bioethanol production cost is much higher than the ethanol market value. Thus, optimizing processing design and increasing by-product value is Khaitan bio energy focus on Zero discharge facility, thus focus on saving our mother planet.
ZLD Ethanol Plants
Compared to the traditional process, this work aims to maximize the value-added by-products and achieve zero waste emission. It proposes a by-products processing path that extracts value-added lignin, furfural and other organics. To realize this, wet stillage undergoes filtering and dried to separate lignin and wastewater in the lignin extraction process. In this process, the insoluble organics such as lignin, small amounts of water and soluble organics will get remove from stillage. The eutrophic wastewater will further extracted to generate furfural, ethanol and other organic powder by multistage fractionation in by-products purification processing and storing in storage. As with traditional bioethanol production, purified water will pump to feedstock pre-treatment processing for water recycling. Except for primary usage, the rest purified water will discharge into the environment. The organic powder will then return to the soil as fertilizer for soil organic matter protection.
To achieve the innovation of the bioethanol production process, the design of the zero liquid discharge process stood on the excellent than previous traditional ethanol production. Therefore, Khaitan bio energy implements the by-products purification process and lignin extraction process focusing towards zero emission. In contrast, the other areas like feed loading, pre-treatment, enzyme production, hydrolysis and fermentation, distillation, utility, and storage.
Regarding environmental impact, CO2 emission is a critical criterion for biorefinery ecological assessment.
The calculation of CO2 emission of electricity is based on the data from the Energy Information Administration (EIA) in 2020 in the US. The total CO2 emission of zero waste emission plant accounts for approximately 27.6 % of that in the traditional bioethanol plant. To extract high-concentration by-products from mixed aqueous solutions, a high volume of water should undergo distillation, resulting in significant electricity consumption in By-product purification.
The zero discharge emission process is more competitive than traditional biorefinery plants. This is mainly in terms of profitability in the ethanol market particularly if possible to achieve low electricity prices. The pre-treatment and fermentation processes are critical in capital cost. High value-added by-products income improves the bioethanol plant’s profitability.
Because the development of lignocellulosic biomass biorefinery is still developing, a substantial technical gap exists in replacing fossil chemicals. Although the purchase price of cellulosic biomass feedstocks is competitive with petroleum on an energy basis, the lack of economic competitiveness in biochemicals is the main challenge for biorefinery.