Coal Mine Sewage: Why the 1980s Activated Sludge Plants Failed and What Replaced Them

Start with the plant layout, not the process

To strengthen coal-mine wastewater treatment and protect the aquatic environment, newly built mines attach great importance to environmental-protection construction and have invested substantial funds, with design units running multi-process, multi-scheme comparisons before anything is poured. Two decisions made early matter more than the technology choice.

The first is whether to build one plant or two. At present some coal mines build a separate sewage-treatment plant for the industrial site and the residential area, which requires two land appropriations, duplicated construction, increased investment, high operating energy consumption, high management costs, scattered technical strength and a high per-ton water-treatment cost. Since the industrial site and the residential area are generally not far apart, joint construction of one sewage-treatment plant of a certain scale is more reasonable. Where draining the residential area to the industrial site would require very deep pipe burying, an intermediate sewage-lifting pumping station can be set up, or the plant can be built on land in the middle area between the two. The joint-build approach saves investment and greatly reduces operating costs.

The second is scale. Many newly built mine designs carry a small labour quota according to codes and overall efficiency, but after actual completion the mine recruits large numbers of contract workers, and with the mine's development a large number of outside personnel arrive, increasing water consumption and therefore sewage volume. A reserve coefficient belongs in the construction scale. At the same time, because coal-mine wastewater quality and quantity vary greatly, reasonably determining design volume and quality bears directly on project investment, operating costs and cost-effectiveness. Production sewage and domestic sewage should be considered together without excessive reserve margin, to avoid increasing investment and leaving equipment idle or running at low efficiency.

Effluent requirements differ considerably among mines and should be determined according to what the environmental-protection authorities require. Because nitrogen and phosphorus in domestic sewage cause eutrophication of water bodies, treatment is required to achieve nitrogen and phosphorus removal.

What mine sewage actually looks like

Coal-mine wastewater is similar in nature to general municipal sewage but differs from it, since municipal sewage often includes some industrial wastewater. Its characteristics can be summarised as: large variation in quality and quantity, low pollutant concentration, good biodegradability, and low treatment difficulty. Low is the operative word, and it is what broke the first generation of plants.

Two decades of failures and fixes

Mine sewage-treatment plants designed in the 1980s mostly adopted the activated-sludge process, and many never worked. Because the organic content of the sewage was too low, the microorganisms could not obtain the minimum nutrients during operation, activated sludge could not form and the plant could not run. The oxidation-ditch process had the same problem: the return activated sludge could not be recirculated, turning the original oxidation-ditch system into an add-on aerated banded horizontal-flow settling tank and failing to meet the required treatment target.

In the 1990s many mines adopted the two-stage biological contact oxidation process for coal-mine domestic sewage, with very good results. It adapts to low-concentration, highly variable mine sewage, saves investment, and is simpler to operate and maintain than the activated-sludge process. Its weak point is nitrogen and phosphorus removal, which is poor. Since the 1990s, research, development and application of new biological wastewater-treatment processes and technologies have achieved great success, and many new processes have emerged, sharing the features of high efficiency, stability, energy savings and multifunctionality including nitrogen and phosphorus removal.

If nutrient removal is part of the duty, the two-stage contact oxidation line will not get there on its own, because it was selected for tolerance of low-strength, highly variable sewage rather than for nutrient removal. The retrofit path is usually to keep contact oxidation as the carbon and ammonia workhorse and add a nutrient-removal stage downstream, rather than rebuilding the whole line.

BAF, and the numbers behind it

Water scarcity has become a global problem and China faces the same reality. Sewage recycling and reuse is one of the effective ways to improve the comprehensive utilisation rate of water resources, ease the contradiction of water shortage, reduce water-body pollution and achieve sustainable use of limited water resources. After coal-mine wastewater is treated and disinfected it can be used for greening, washing and industrial water.

Using the biological aerated filter (BAF) process to treat coal-mine wastewater gives stable effluent quality, superior to general traditional biological treatment processes; after disinfection the effluent can be reused as reclaimed water. BAF features small volume, land savings, high efficiency, good effluent quality, simple flow and convenient operation and management. In actual operation it can achieve centralised control and on-site manual or automatic control, and after multiple engineering applications it has become increasingly mature, with effluent meeting reclaimed-water reuse standards after disinfection.

The economics are the argument. Direct investment per m3 of sewage is about 1,000 yuan with conventional approaches, while the BAF process can control it at about 500 yuan and saves nearly 4/5 of the footprint. Given that coal-mine wastewater varies greatly in quality and quantity, has low pollutant concentration and biodegrades readily, BAF is quite suitable.

Sequencing the build

Sewage pipe networks and sewage-treatment plants should be built in phases and batches according to the mine's overall plan and drainage plan, implemented phase by phase in line with water-environment protection goals and gradually brought up to standard. Mines that build everything at once on day-one assumptions, before the workforce and water consumption have stabilised, end up with oversized assets and a per-ton cost that never recovers.