Pilot-scale treatment of hypersaline coal chemical wastewater with zero liquid discharge

Highlights

• A pilot-scale ZLD approach was established to treat coal chemical wastewater.

• Integrated process includes membrane, softening, oxidation, and evaporation units.

• The proposed ZLD system achieved around 98% water recovery.

• Industrial-grade salts of Na₂SO₄ and NaCl are obtained by the proposed ZLD process.

• The operating cost of overall proposed ZLD process is about 2.3 $ m⁻³ influent.

Abstract

Coal to chemical processes produce substantial quantities of hypersaline organic wastewater. Zero liquid discharge (ZLD) is a strategy designed to minimize the wastewater discharge. Here, a pilot-scale ZLD system, consisting of nanofiltration (NF), disc tube reverse osmosis (DTRO), chemical softening, electro-oxidation (EO) and multiple-effect evaporator/crystallizer, was built for the treatment of coal chemical wastewater. The hybrid ZLD process is capable to treat 72 m³ influent per day and reached about 98% water recovery. First, bivalent and monovalent ions were separated from the raw water by a NF-DTRO process. Then the hardness and organic substances of the wastewater were removed by the chemical softening and the EO techniques. Eventually, industrial-grade salts of Na₂SO₄ and NaCl were recovered through an evaporator-crystallizer. The total operating cost is estimated to be 2.3 $ m⁻³ influent. This pilot-scale ZLD system is applicable to the treatment and reuse of hypersaline wastewater. The work will provide insights in large-scale application of the ZLD process for wastewater treatment.

Introduction

Coal is an essential raw material to many chemicals and fuels [1]. However, in the coal-to-chemical industry, converting 1 ton of coal to other chemicals may produce 1 m³ of hypersaline organic wastewater, which poses a serious threat to water security [2]. Besides inorganic ions, coal chemical wastewater contains a large amount of hazardous substances such as ammonia nitrogen, polycyclic aromatic, and phenols etc. [1]. Zero liquid discharge (ZLD) is a wastewater management strategy that advocates minimizing wastewater discharge and maximizing water reuse. To realize ZLD, advanced water treatment techniques are integrated to purify and reuse wastewater, as well as recover valuable byproducts.

Ultrafiltration (UF) and reverse osmosis (RO) integrated process is a mature technology that has been successfully applied in the reuse of hypersaline industrial wastewater in coal chemical industry, in which the UF process functions as a pretreatment step for removal of suspended particles and colloids, while the RO membrane rejects salt ions and organic contaminants [3], [4]. However, high-concentration organic contaminants and severe concentration polarization (CP) causes fouling/scaling of the RO membrane, especially in conventional spiral-wound configuration; this leads to a reduction in water recovery and requires frequent membrane cleaning, eventually hindering the efficiency and reliability of the system [5]. Compared to the spiral-wound configuration, the disc-tube RO (DTRO) provides shorter flow channel and more uniform flow distribution, thus generating higher turbulence and inhibiting CP effects along the membrane surface. The DTRO is hence more appropriate for treating hypersaline organic wastewater [6], [7], in which the water recovery can be elevated to 80% with a permeate flux of 34 L m⁻² h⁻¹ [8]. The flux of DTRO system can be easily recovered with periodic water flushing in long-term operation. In addition, an electrochemical oxidation (EO) process without addition of any other chemicals that oxidizes organic containments using electrochemically-generated hydroxyl radicals [9], was employed for removing ammonia nitrogen and COD (>90%) in the RO concentrate of coal gasification wastewater and coal tar wastewater under mild current density (30–50 mA cm⁻²) [10], [11].

Besides, other techniques are utilized for recovering salts from the RO concentrate stream to achieve maximum water reuse. NF has a unique characteristic of selective separating multivalent ions and organic compounds from monovalent ions [12]. Thus, a NF unit is capable of removing most chemical oxygen demand (COD) and sulfates (e.g. rejection rate of >99% for sodium sulfate) in the highly concentrated RO brine, only leaving chlorides (e.g. rejection rate of <20% for sodium chloride) in the permeate [13]. In order to ensure the purity of recovered salts, a pretreatment technique for completely eliminating hardness and organics in the NF concentrate is demanded prior to the evaporative crystallization process [14]. Chemical precipitation is an effective softening approach to remove the mineral scalants, as well as other trace constituents (e.g., strontium and silica et al.) via co-precipitation processes [15]. Even though the aforementioned technologies have been established, the technical and particularly economic feasibility of their industrial-scale integration have not been demonstrated so far.

The key objective of this study is to construct a pilot system for treating the RO concentrate from coal chemical industry at a maximum rate of 3 m³ h⁻¹ for water reclamation and salt recovery (i.e., NaCl and Na₂SO₄). To achieve ZLD condition, the treatment chain includes nanofiltration (NF), disc tube reverse osmosis (DTRO), chemical softening, electro-oxidation (EO), and evaporation-crystallization. We experimentally test the filtration performance and stability of the proposed hybrid system, assess the fouling tendency of the DTRO and NF modules, and provide the energy and cost analysis in a long-term operation. The outcome of this study would serve as a valuable guide for ZLD process design and practical operation in subsequent large-scale industrial applications.