What Is Zero Discharge? (Zero Liquid Discharge, ZLD)
In terms of its content, zero discharge means, on the one hand, controlling the emission of unavoidable waste generated during production and reducing it to zero; and on the other hand, making full use of unavoidable emissions so as to ultimately eliminate the existence of non-renewable resources and energy. In terms of process, it refers to transforming the waste discharged from one industrial production process into the raw material or fuel of another industry, so that related industries form an industrial ecosystem through recycling. From a technical perspective, in industrial production the conversion of energy, power, and resources follows certain natural laws; the conversion of resources into various forms of energy, the mutual conversion of various energies, and the conversion of raw materials into products can never achieve 100 percent efficiency. According to the laws of conservation of energy and conservation of matter, the lost portion is ultimately discharged into the environment in the form of water, gas, noise, residue, and heat.
Because environmental protection in our country started relatively late, achieving a true reduction of unavoidable waste emissions to zero under existing technological and economic conditions is extremely difficult. Some enterprises, through the full utilization of unavoidable waste, have achieved so-called zero discharge, but this only changes the mode, channel, and node of pollutant emission; some pollutants will eventually enter the environment. In this sense, true zero discharge is only a theoretical, ideal state.
Zero-discharge technology comprehensively applies physical, chemical, and biochemical processes such as membrane separation, evaporative crystallization, and drying to concentrate the solid impurities in wastewater to a very high concentration. Most of the water is recycled, while the small remaining amount of water accompanying the solid waste can be directed to one of the following outlets according to each enterprise's specific situation, without being discharged from the system (such a zero-discharge decision should at least consider three major factors: environmental requirements, economic cost, and production safety).
Solar evaporation ponds for natural evaporation
Used to produce by-products and enter solid products
Sprayed into an incinerator for waste treatment
Absorbed by solid waste (e.g., fly ash) and treated as solid waste
Starting from the 1970s, individual industrial sectors began exploring zero discharge, which at that time mainly meant that no wastewater was discharged from the factory; all wastewater underwent secondary or tertiary treatment, and apart from reuse only solid residue remained. It was not until 1994, when Belgian entrepreneur Gunter Pauli founded the Zero Emissions Research Initiatives (ZERI) in Belgium, that zero discharge was elevated from isolated, scattered activities into a theoretical system. In 1998, the United Nations officially recognized the concept of zero discharge and began pilot projects in cooperation with the ZERI foundation. In 1999, the UNU/Zero Emissions Forum was established at the United Nations University headquartered in Japan. In 2007, this forum cooperated with the Resource Conservation and Environmental Protection Department of China's National Development and Reform Commission to hold a forum in Beijing on Developing a Circular Economy and Promoting Zero Waste Emission.
At the end of August 2011, Beijing's first zero-discharge courtyard house (siheyuan) was completed in the Dongcheng District Daxing Community Residents' Committee. As a representative of Beijing culture, making the siheyuan zero-discharge is not only a beneficial practice for the protection and development of the courtyard house, but also provides an example for the renovation and protection of Beijing's courtyard houses. The zero-discharge campaign for courtyard houses must seek an optimal balance among inheriting traditional culture, enjoying a healthy and comfortable modern lifestyle, and low-carbon energy conservation. The reason for creating zero-discharge courtyard houses is that the siheyuan is a representative of Beijing culture. As a unique architectural form of old Beijing, its layout and design have been refined over hundreds of years and perform excellently in adapting to Beijing's specific climatic conditions, but may still fall short of meeting modern habitability requirements. If, on the basis of preservation and inheritance, and while still adhering to the concept of small investment and big environmental protection, we reasonably apply low-carbon technologies to reduce energy consumption and achieve a comfortable modern lifestyle, then old Beijing's courtyard houses can also glow with new green. [1]
Given that what is currently discussed as zero discharge in our society mainly refers to the original meaning of zero wastewater discharge, abbreviated as ZLD (Zero Liquid Discharge).
