The Trap of “Intermittent Exceedances” in 3 Categories of Wastewater Discharge Monitoring Indicators at Packaging Printing Plants—How Printing Plants Are Most Likely to Fail Inspections on pH and COD During Surprise Visits by Environmental Protection Officials
💡 💡 At a Glance
The three monitoring indicators most likely to cause printing facilities to fail wastewater discharge inspections are PH (6-9), COD (≤100 mg/L), and SS (≤70 mg/L). PH and COD are especially prone to intermittent exceedances when color-adjustment sinks and ink-change cleaning tanks are washed. Printing facilities should install two sets of buffer systems: a PH adjustment tank with an emergency neutralization system, requiring an investment of 80,000-150,000 Yuan, and an accident emergency tank with COD pretreatment equipment, requiring an investment of 250,000-500,000 Yuan. Compared with potential fines of 50,000-200,000 Yuan per incident and production shutdowns lasting 3-7 days, these investments can be recovered within 1 year.
In June 2024, a printing plant in Zhongshan, Guangdong, that produces cosmetic folding cartons was subjected to an unannounced inspection led by the municipal Ecology and Environment Bureau. At 10:30 a.m., the plant was changing inks and cleaning the ink-washing tank. The on-site pH monitoring instrument displayed 11.2, while the COD reading was 920 mg/L, exceeding the Level 1 limits of the Integrated Wastewater Discharge Standard (GB 8978-1996), which require a pH of 6–9 and COD ≤ 100 mg/L. The director issued a penalty on the spot and fined the plant 120,000 yuan. This was the printing plant owner’s worst nightmare: third-party testing had shown compliant results every month, but the 24-hour continuous records from the online monitoring instruments exposed intermittent violations.
The Most Common Misconceptions That Trap Printing Shop Owners
Many printing plant owners believe that wastewater discharge simply means “having a third-party organization collect a water sample once a month and issue a report confirming that it meets the standards.” In reality, since 2020, the primary regulatory tool used by local ecological and environmental authorities to oversee the printing industry has been the online monitoring instrument (also known as the automatic pollution source monitoring system), with data transmitted directly to the environmental authority’s backend system every 5 minutes. Third-party sampling and monitoring can only address past conditions, while online monitoring is essential for day-to-day oversight.
The two periods when printing plants are most likely to exceed limits are: first, ink color adjustments and ink changes that require cleaning the wash trough, typically from 9-11 in the morning and 2-4 in the afternoon, when alkaline cleaning agents (sodium hydroxide solution) cause the pH to rise instantly above 10; and second, concentrated plate and roller washing, typically when changing orders, when ink, starch, and resins are washed away together, causing the COD to surge to 500-1500 mg/L. These intermittent exceedances during these two periods are unlikely to be detected in monthly third-party sampling, especially if the sampling time happens to avoid them. However, online monitors record conditions 24 hours a day, so the environmental protection authority can identify the exceedances as soon as it retrieves the data.
The Real Data Boundaries of the Three Types of Monitoring Indicators
Three categories of core indicators for wastewater discharge monitoring in the printing and packaging industry are essential:
- pH value (GB 8978 Class I standard 6-9)—The pH fluctuations in wastewater from cleaning operations at printing plants are the greatest. The pH of sinks in offset printing workshops typically ranges from 8-10, while that in water-based ink workshops typically ranges from 9-11. In UV ink workshops, however, the pH of sinks is closer to neutral. pH exceedances result in the most frequent fines, accounting for 40% of wastewater penalties issued in the printing industry.
- COD (chemical oxygen demand) (GB 8978 Class I standard ≤ 100 mg/L)—COD is a comprehensive indicator that accounts for all organic pollutants. COD in printing plant wastewater typically ranges from 200-1500 mg/L, mainly originating from inks, starch, resins, and surfactants. Fines for exceeding COD standards can reach 50%, making this the largest penalty category.
- SS (suspended solids) (GB 8978 Class I standard ≤ 70 mg/L)—SS mainly comes from paper fibers, ink residue, and coating flaking. SS in printing plant wastewater is typically 100-500 mg/L and can be reduced to 30-50 mg/L after filtration.
In addition to these 3 categories, some regions also monitor ammonia nitrogen (NH3-N, Class I standard ≤ 15 mg/L), total phosphorus (Class I standard ≤ 0.5 mg/L), total nitrogen (Class I standard ≤ 20 mg/L), and colority (≤ 50 times). When a printing plant owner undergoes an environmental inspection for the first time, it is essential to confirm with the local ecology and environment bureau whether the applicable standard is Class I or Class II. Under the Class II standard, the pH must be 6-9 and COD must be ≤ 150 mg/L, and the fine amount may differ by 30%.
Data Patterns of “Intermittent Exceedances” for the Three Types of Indicators
Print shop owners need to understand how these 3 types of indicators fluctuate throughout the day to take targeted preventive measures:
- Peak pH: 9-11 a.m. (ink color adjustment, ink change, and water trough cleaning); 2-4 p.m. (order changeover and equipment cleaning); stable after 8 p.m.
- COD peak: Concentrated within 30 minutes after each plate washing and car washing, with the peak lasting 1-2 hours; without plate washing, COD is usually ≤ 200 mg/L.
- SS peak: paper debris and ink residue emissions can form impulsive peaks lasting 10-30 minutes.
The lesson from a printing plant in Zhongshan is clear: when online monitoring equipment is installed only at the final discharge outlet, pH and COD levels spike during cleaning operations, allowing the environmental protection authority to retrieve the data and impose penalties. If a printing plant installs online monitoring equipment only at the final discharge outlet, intermittent exceedances will be detected 100% of the time.
2 Essential Cushioning Systems for Printing Plants
First system: pH adjustment tank + emergency neutralization unit—The pH adjustment tank must accommodate the wastewater from one complete cleaning cycle, typically 5-10 cubic meters. Install an automatic pH dosing system in the tank for sulfuric acid or sodium hydroxide, and discharge the wastewater only after adjusting its pH to 7-8. The emergency neutralization unit provides rapid response when the pH suddenly rises to 11, and the sulfuric acid reaction time must be controlled within 5 minutes.
Second system: Accident emergency pond + COD pretreatment unit—the accident emergency pond typically has a capacity of 1.5-2 times the average daily wastewater volume (15-25 cubic meters) to buffer COD peaks during plate washing and vehicle washing. The COD pretreatment unit includes: a dissolved air flotation unit (to remove ink and resin), a filter (to remove paper scraps and ink residue), and activated carbon adsorption (to reduce COD). The dissolved air flotation unit costs 80,000-150,000 yuan, the activated carbon adsorption tower costs 30,000-80,000 yuan, and monthly operating costs range from 2,000-5,000 yuan.
Another 2 cost-saving tricks are to stagger cleaning times, avoiding concentration during regulatory peak periods, and to separate wastewater from different workshops, collecting adhesive printing workshop wastewater, UV workshop wastewater, and domestic sewage separately, pretreating each stream before combined discharge. These measures cost nothing and require only proper scheduling.
Cost-Effectiveness Analysis: Is This Eco-Friendly Investment Worth It?
A pH adjustment tank, emergency neutralization system, accident emergency tank, and COD pretreatment system require a total investment of approximately 250,000–500,000 yuan. Monthly operating costs are 5,000–10,000 yuan, equivalent to 1.5–2.5% of the printing plant’s annual revenue. Compared with potential fines of 50,000–200,000 yuan per incident and losses from corrective shutdowns lasting 3–7 days, the investment pays for itself within 1 year.
When a printing plant owner first receives a quotation from an environmental equipment supplier, the full system price of 800,000-1,000,000 yuan can be alarming. In reality, the two essential components are the pH adjustment tank and the emergency accident pool. Other COD/SS treatment equipment can be introduced in stages: first install the most affordable filter machine (20,000-30,000 yuan), observe the operating data, and then decide whether to add a dissolved air flotation unit and activated carbon system.
Further Reading
FAQ
How much is a printing plant typically fined for exceeding wastewater discharge standards?
Under Article 83 of the Water Pollution Prevention and Control Law, violations of discharge standards are subject to fines of 100,000–1,000,000 Yuan; in serious cases, the facility may be ordered to suspend production for rectification. In actual enforcement, fines for printing plants exceeding PH and COD limits typically range from 50,000 to 200,000 Yuan. When combined with lost orders during the 3–7 days required for rectification and production suspension, the total cost can reach 300,000–500,000 Yuan per incident.
Is an online monitoring device mandatory? Can it be omitted?
In principle, the “Measures for the Administration of Automatic Monitoring of Pollution Sources” require key pollutant-discharging units to install such systems. Whether a printing plant qualifies as a “key pollutant-discharging unit” depends on the determination of the local ecological and environmental authority. Printing plants with an annual wastewater discharge of more than 5000 tonnes, those that use toxic or hazardous raw materials, and those located in sensitive waters are generally classified as key units. Failure to install the system may result in being listed as operating pollution-source monitoring facilities improperly, with fines of 50,000–200,000 Yuan.
Must a printing plant treat its wastewater on its own, or can it outsource treatment to a third party?
A third-party treatment company may be engaged, but the printing plant remains the responsible party. The third-party treatment company must hold a Hazardous Waste Business License or wastewater treatment operation qualification, depending on the nature of the wastewater. When the printing plant and the third-party company sign a hazardous waste disposal agreement, they must clearly define each party’s responsibilities and provide the third-party company with the wastewater composition list (MSDS). The fee for outsourced treatment is typically 80-200 Yuan per ton, accounting for 2-3% of the printing plant’s annual revenue.
How is the capacity of a pH adjustment tank calculated?
<p>Empirical formula: pH adjustment tank capacity = maximum cleaning wastewater volume per operation × 1.5. In the offset printing workshop, a single ink-mixing or color-change cleaning operation generates approximately 1-2 cubic meters of wastewater. In the UV workshop, a single press-washing operation generates approximately 2-3 cubic meters of wastewater, while the plant-wide peak demand is approximately 5-8 cubic meters. The recommended pH adjustment tank capacity is 8-12 cubic meters, providing a 50% reserve margin.</p>
How much does a COD pretreatment system cost, and how long does it take to recoup the investment?
An air flotation machine costs 80,000–150,000 Yuan, a filter costs 20,000–30,000 Yuan, and an activated carbon adsorption tower costs 30,000–80,000 Yuan, bringing the total equipment investment to 130,000–260,000 Yuan. Monthly operating costs are 2,000–5,000 Yuan, covering chemicals, electricity, and labor. Compared with fines of 50,000–200,000 Yuan per inspection, if a printing plant has a 50% chance of being inspected once a year, the investment will pay for itself in 1.5–2 years. If the plant is inspected more than twice, the investment will be recovered within the same year.
How can printing factory owners determine for themselves whether their wastewater exceeds the permitted limits?
The simplest approach is to install a portable pH meter (200-500 Yuan) and a rapid COD tester (1500-3000 Yuan), take measurements three times a day, and record the data in a log. If the pH remains close to 9 or the COD remains close to 90 for 3 consecutive days, immediately check the operating condition of the pH adjustment tank and the emergency accident tank. This “owner self-inspection” costs less than 5000 Yuan per year and can prevent 80% of sudden exceedances.
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