Brief Discussion on Chemical Treatment Methods of Reclaimed Water in Circulating Cooling Systems

With increasing urban water scarcity, more municipal and industrial wastewater is being deeply treated and reused as make-up water for circulating cooling systems [1]. Reclaimed water is characterized by complex water quality, high suspended solids content, and nutrient richness. When such water is reused in circulating systems, the primary concern lies in the accumulation of harmful ions. As the water is continuously recycled and concentrated, these ions multiply, significantly increasing the risks of corrosion and scaling.

In industrial circulating water treatment, the quality of make-up water plays a pivotal role in determining the system's corrosion and scaling tendency. This, in turn, is closely tied to the selection and application of corrosion inhibitors, scale inhibitors, and biocides. Let's explore this topic further with insights from reclaimed water equipment manufacturers.

Currently, most research focuses on the process design of advanced treatment for reclaimed water or individual treatment units. However, few studies address the specific chemical treatment methods required when reclaimed water is used as make-up for cooling systems. This paper proposes a new approach to solving the corrosion and scaling problems caused by reclaimed water, based on the operational experience of existing systems. It introduces optimized dosing strategies and adjusted chemical formulations.

2.1Formulation of a Composite Corrosion and Scale Inhibitor

Ingredients by weight:

• Polyepoxysuccinic acid: 9–11 parts
• 2-Phosphonobutane-1,2,4-tricarboxylic acid (PBTCA): 18–22 parts
• Dihexamethylene triamine penta(methylene phosphonic acid) (DTPMPA): 19–21 parts
• Self-made dispersing scale inhibitor: 25–35 parts
• Benzotriazole: 1–3 parts
• Deionized water: 9–28 parts

(1) Preparation of self-made dispersing scale inhibitor:

Composed of:

• Maleic anhydride: 480–510 parts
• Sodium dihydrogen phosphate solution: 200–230 parts
• Initiator (e.g. potassium persulfate): 100–120 parts
• Deionized water: 290–310 parts
• Acrylic acid: 178–185 parts

Preparation method:
Add deionized water and maleic anhydride into the reactor with stirring until dissolved. Then slowly drip in acrylic acid, sodium dihydrogen phosphate solution, and the initiator over 2.8–3.0 hours, maintaining the temperature at 60–70 °C. After all components are added, keep stirring at 68–72 °C for 3–4 hours until a transparent viscous liquid is obtained. The resulting product has a solid content of 30–50%.

• Sodium dihydrogen phosphate solution: Prepared by dissolving 45 parts sodium dihydrogen phosphate in 155 parts deionized water.
• Initiator solution: 10–15 parts potassium persulfate dissolved in 100 parts deionized water.

(2) Preparation of the composite corrosion and scale inhibitor:

Add polyepoxysuccinic acid into PBTCA and mix thoroughly. Then successively add DTPMPA, self-made dispersing scale inhibitor, and deionized water while maintaining 20–35 °C. Stir for 25–35 minutes. Finally, add benzotriazole and continue stirring for 30–60 minutes to complete the formulation

2.2 Microbial Control Strategy

Microbial control adopts a combination of oxidative biocides, non-oxidative biocides, and biological dispersants:

• Oxidative biocides: Sodium dichloroisocyanurate, trichloroisocyanuric acid, or bromochlorodimethylhydantoin (BCDMH), with BCDMH being preferred. Dose: 20 mg/L, twice weekly.
• Non-oxidative biocides: Modified quaternary ammonium compounds. Dose: 100 mg/L, once every two weeks.
• Biological dispersant: Nonionic surfactants. Dose: 20 mg/L, once weekly.
• All microbial control agents should be dosed manually in high-flow areas of the collection tank based on the total water volume.

2.3 Circulating Water System Optimization

The system uses the SYZL ∑sigma automatic total phosphorus online monitoring and dosing system to regulate both chemical dosing and water make-up/discharge. The composite corrosion and scale inhibitor is dosed at 10–15 mg/L based on make-up water volume. Total phosphorus in circulating water is maintained at 4–7 mg/L, with a concentration ratio of around 5.0. Side-stream filtration is applied, and a QYC-120 heat exchanger monitor is installed.

All chemicals except the custom-made dispersant are commercially available industrial-grade products. The dispersant has a solid content of over 30%.

The water sample used is shown in Table 1, and the test method follows HG/T 2160-2008 "Dynamic Simulation Test Method for Cooling Water" [2]. Results are presented in Tables 1 and 2.

As shown in Table 2, the composite corrosion and scale inhibitor demonstrated excellent performance in reclaimed water-based cooling systems, effectively controlling both scaling and corrosion. The corrosion rate remains well within the limits specified in GB 50050-2007.