Biological contact oxidation and an artificial floating island for black odorous river purification Open Access

Accelerated industrialization and rapid economic development in China have rapidly deteriorated the environment, intensified water pollution, and caused serious ecological damage, among other problems (Wang & Yang 2016). In China, numerous urban rivers seasonally or permanently turn black and carry an unpleasant smell, posing a severe water problem (Ji et al. 2017; Zhang et al. 2019). According to the data released by China's Ministry of Housing and Urban–Rural Development–Ministry of Ecology and Environment in October 2021, the total number of black odorous water bodies in China is 2,869, of which 556 are under treatment; however, there are still black odorous rivers that continue to pollute the environment (Ministry of Housing and Urban–Rural Development of China 2022). Compared with normal rivers, black odorous water bodies are typically characterized by a low dissolved oxygen (DO) content, oxidation–reduction potential (ORP), and pH, as well as high nitrogen and phosphorus levels. For example, the DO content is mainly maintained between 0 and 1 mg/L, NH₃-N levels vary from 2 to 50 mg/L, and total phosphorus (TP) levels may exceed 17 mg/L (Cao et al. 2020). The reasons for the black color of these water bodies include the consumption of oxygen by organic matter, low ORP of the water body, and presence of metal sulfides (Rong et al. 2020). The unpleasant smell is mainly caused by volatile organic sulfides, hydrogen sulfide, and various algal metabolites (Cao et al. 2020). Remediation of black odorous water bodies can be divided into physical, chemical, and bio-ecological remediation techniques (Xie et al. 2020). Artificial aeration, water cycling (artificially connecting rivers or lakes to make water flow and cycle), and substrate dredging are physical remediation techniques, whereas chemical remediation techniques include chemical flocculation, chemical sedimentation, and chemical algal removal. Compared with traditional physical and chemical remediation methods, bio-ecological restoration is more effective at removing organic matter and has a lower impact on the environment, making it an appropriate long-term strategy (X. Wang et al. 2020).

Being an effective ecological restoration method for water purification, artificial floating island (AFI) technologies have been widely used in rivers, lakes, and reservoirs for 40 years in China (Kong et al. 2019). An AFI is a soilless planting structure composed of floating bodies, aquatic plants, and the associated microbial communities (Yeh et al. 2015). AFIs have various functions, including absorbing nitrogen and phosphorus from water through plants, intercepting suspended materials, providing microbial attachment vehicles, reducing light penetration, and inhibiting algal growth (Lu et al. 2015). However, the coverage of AFIs may cause hypoxia in water bodies, the use of non-native plants may lead to species invasion, and decaying plants and floating island materials may become a source of pollution. Research has shown the combination of aeration and AFIs can prevent suspended substrates from sinking, strengthen the contact between microorganisms and dissolved oxygen, and help eliminate water pollutants, such as organic matter, nitrate nitrogen (NO₃-N), and phosphate ions (Liu et al. 2014).

The biological contact oxidation (BCO) process, also known as the integrated biofiltering or contact aeration process, is a hybrid wastewater treatment system that combines the advantages of the activated sludge process and biofilm method (Ateia et al. 2016). In this process, pollutants are absorbed and degraded by microorganisms on biofilm attached to a carrier, thereby purifying the water. Owing to its tolerance to high NH₃-N levels and strong resistance to shock loading, this process is widely used for treating organic wastewater (Li et al. 2021). The basis of the BCO process is the selection of a suspension carrier filler that directly affects the water quality and operation management of BCO (Tang & Sun 2018). Traditional BCO tanks use continuous aeration to remove organic matter and NH₃-N; however, continuous aeration has the disadvantages of high energy and carbon consumption. In addition, continuous aeration is not conducive to denitrification, resulting in low nitrogen removal efficiencies. Moreover, intermittent aeration can alter the environment and improve pollutant removal (Zhang et al. 2021).

Owing to the unclarified mechanism of water blackening and odorization, a single technique cannot ensure the purification and long-term remediation of black odorous water bodies (Fan et al. 2019). Herein, a combination of source control, substrate purification, composite AFI, improved BCO, and water ecosystem reconstruction was used to treat a severely polluted black odorous river in Dongguan City, Guangdong Province. The water quality before treatment was as follows: transparency of 3–7 cm, DO content of 0.1–0.4 mg/L, NH₃-N of 15.22–19.27 mg/L, TP content of 1.66–2.28 mg/L, and ORP of −261 to −294 mV. We require the quality of the water bodies to reach standard V at the surface, with transparency >25 cm and ORP >50 mV, to ensure that the black odor phenomenon is eliminated, pollutants are substantially removed, and the environment is improved.

The river not only has a dark color and low transparency, but also some garbage floating on the surface, a large number of plants growing on both banks of the river, and a distinctly unpleasant smell. Sampling was conducted at monitoring sites S1, S2, S3, and S4 (Figure 1) every 10 d from 9:00 to 10:00 AM from October 2020 to April 2021 for water quality monitoring (Table 1). The samples were collected at a 0.5 m depth underwater in the middle of the river of the monitoring point and were immediately sent to a laboratory to measure the NH₃-N and TP contents. The transparency, DO content, pH, and ORP were measured in-situ, and sampling was conducted three times and the average was calculated. According to the Working Guidelines for the Treatment of Urban Black-Odorous Water 2015 (Working Guidelines for the Treatment of Urban Black-Odorous Water 2015) and Environmental Quality Standards for Surface Water (GB 3838-2002) (Environmental Quality Standards for Surface Water (GB 3838-2002)), the transparency, DO content, and ORP of the river were very low, while the NH₃-N and TP values exceeded standard V at the water surface by approximately eight and five times, respectively. This indicates that the river was a severe black odorous water body that failed to meet standard V at the water surface before treatment.

The water quality analysis included measuring the transparency; DO, NH₃-N, and TP contents; and ORP. Water quality detection was conducted according the Analytical Methods for Water and Wastewater Monitoring 2002 (4th edition) (State Environmental Protection Administration 2002) released by the Chinese State Environmental Protection Administration and the Standard Methods for the Examination of Water and Wastewater 1998 (20th edition) (APHA/AWWA/WEF 1998) published in the USA. The testing methods and instruments used are shown in Table 2.

River sediment, which can exchange material and energy with water as well as adsorb and degrade certain pollutants, plays an important role in self-purification (Zhang et al. 2019). However, the large amounts of pollutants deposited at the bottom of a river weaken the river's self-purification capacity, and nutrients such as nitrogen and phosphorus in the sediment are easily released into the water column, resulting in secondary pollution (Z. Wang et al. 2020). Dredging is a commonly used substrate treatment method and achieves complete removal of pollutants; however, the removed substrate needs further treatment, which is costly and creates secondary pollution. (Wang et al. 2019). In recent years, substrate in-situ remediation technologies have been widely used to remediate black odorous rivers. In this study, a substrate improver with zeolite, magnesium-aluminum hydrotalcite, calcium peroxide, polymeric aluminum chloride, and other materials was used to remediate black odorous water bodies by reducing the capacity of substrates, removing nutrients, and fixing heavy metals. This would activate indigenous microorganisms and restore the ecological chain.

Shipboard machinery was used to stir and convert the substrate into a suspended state, and then the substrate improver was added. The surface area of the river is approximately 12,600 m², the depth of the substrate is approximately 0.3–0.5 m, and the amount of substrate improver used was 0.5 kg/m². Five days after the first dosing, the sediment was stirred again in the same manner, and dosing was conducted every five days for 20 d.

Artificial floating islands (AFIs) mainly purify water bodies by absorbing nutrients, such as nitrogen and phosphorus, in water from cultivated aquatic plants, while microorganisms decompose organic matter on the biofilm formed by plant roots (Yang et al. 2021). However, due to the limited plant area, the pollutant removal rate of traditional AFIs is inefficient. Most aquatic plants with shallow roots on floating islands can only remove pollutants from the water surface; thus, restoration of deep-water bodies is limited. Meanwhile, the very low dissolved oxygen content of black odorous water bodies is harmful to the growth of plants and affects the activity of microorganisms (Lv et al. 2019). This study improves the traditional AFI in the following ways: (1) Additional aeration pipes for aeration placed at the bottom of black odorous rivers can increase the low dissolved oxygen content in the water column and change the state of suspended matter. (2) Modified fiber balls suspended under a floating island increase the adsorption capacity of organic matter and utilize microorganisms to remove pollutants from deep water bodies. (3) T. dealbata and M. elatinoides, which have strong pollution resistance among native plants, were selected for planting. As the two plants have different rooting depths, they not only cooperate during river restoration, but also grow and reproduce rapidly. (4) Aerobic microbial strains, including nitrifying bacteria and Bacillus subtilis, were chosen to accelerate film hanging.

Traditional BCO ponds are easily clogged and exhibit low nitrogen and phosphorus removal rates, making them unsuitable for the remediation of black odorous rivers. In this study, the traditional BCO pond was improved as follows: (1) The shape and design of the conventional BCO basin was altered to suit the river. (2) New filler combinations were adopted to increase the pollutant removal rate. (3) The flow direction was transformed into a folded flow to increase the contact and hydraulic retention time of the water body and filler. (4) The aeration mode was changed – intermittent aeration was used to hang the biofilm to create alternating anaerobic, anoxic, and aerobic conditions, which is useful for increasing the nitrogen and phosphorus removal rates. (5) Compound microbial strains were added to increase efficiency of nitrogen and phosphorus removal and shorten the reactor start-up time.

Water ecological reconstruction refers to the adoption of engineering and non-engineering measures to promote the restoration of water ecosystems to a more natural state. Plants play an important role in water purification and ecosystem restoration by absorbing and assimilating excess nitrogen and phosphorus in water (Yu et al. 2019). The removal of nitrogen and phosphorus occurs mainly because of the synergy between plant roots and microorganisms, and only a few are directly absorbed by plants (Su et al. 2019). Studies have shown that combining aquatic plants and aquatic animals helps to remove pollutants, stimulates the growth of aquatic plants, and improves water transparency (Gao et al. 2017).

Within 300 m behind the modified BCO pond, submerged plants Vallisneria natans (Lour.) Hara, Ceratophyllum demersum L., and M. elatinoides were planted, together with emergent plants Nelumbo nucifera Gaertn. and Canna indica L., covering an area of approximately 2,400 m² and accounting for 19.1% of the water surface. The compound microbial strains were added, including nitrifying bacteria, denitrifying bacteria, and B. subtilis. The concentration of bacterial solution was approximately 1.0×10⁹ CFU/mL, the dose was 0.5 L/m³, and the total dose was approximately 1,800 L. Fish can clean up phytoplankton, while mussels can filter microscopic organisms and organic debris suspended in the water and Sinotaia quadrata secretes pro-flocculating substances, and the presence of the appropriate aquatic animals, including Cyprinus flammans (Richardson), Carassius auratus, Sinotaia quadrata, and Anodonta woodianawoodiana, all of which are native species, can improve the transparency of water. The density of the fish casting was approximately 20–40 fish/100 m², with 30–50 g/strip and a total casting of approximately 1,200–2,600; the density of the Sinotaia quadrata casting was approximately 5–10 strip/m², with 2–10 g/strip and a total casting of approximately 32,000–64,000; the density of the Anodonta woodianawoodiana casting was approximately 1–2 strip/m², with 10–15 g/strip and a total casting of approximately 6,400–12,800.

The system was built from April to June 2021 and began operating stably in July. According to the construction sequence, the system was divided into five parts: stage I – construction of substrate purification, an ecological cofferdam, and an ecological dam; stage II – construction and operation of a BCO tank; stage III – construction of a composite AFI; stage IV – construction of a water ecosystem; and stage V – stable operation of the system. The water transparency, DO, NH₃-N, and TP contents, and ORP were sampled at monitoring points S1, S2, S3, and S4 every 10 d during the construction and operation of the system. Among the four sampling points, S1 is the water inlet point, between S1 and S2 is the composite AFI, between S2 and S3 is the improved BCO tank, between S3 and S4 is the water ecosystem, and S4 is the final water outlet of the system.

The water transparency, DO content, and ORP were improved significantly through the composite AFI. An NH₃-N removal rate of 30.58% was reached, and the TP content reached 38.30% and accounted for the largest percentage (Table 3). This indicates that the composite AFI efficiently removes pollutants (phosphorus in particular). This is because phosphorus-accumulating bacteria, which always maintain high activity due to continuous aeration, absorb large amounts of phosphorus, and phosphorus in the water body is removed when large quantities of growing and reproducing plants are harvested. The improved BCO process had the greatest improvement on water transparency, DO content, and the ORP, and achieved the highest removal rates of 43.08% for NH₃-N while the removal rate of TP was 28.19%, respectively, indicating that it is beneficial for the purification of black odorous water bodies. The improved BCO process had the highest NH₃-N removal efficiency because intermittent aeration creates alternating anaerobic, anoxic, and aerobic conditions in the water body, and nitrogen elements are transformed into nitrogen for elimination. Moreover, source control improves water quality to an extent and reduces the load on the water body. Finally, the construction of a water ecosystem is important for ensuring water quality and improving the appearance of the environment.

Construction, aeration, and plant harvesting was performed for four months to stabilize the system. Long-term water quality monitoring was performed within six months of stabilizing the system, with samples taken at 10 d intervals for testing (Table 4). The results show that the effluent met the expected target, eliminated black odor, and met standard V for surface water.

The total water surface area of the river is approximately 13,680 m². We made a rough assessment of the cost of the system, including engineering and construction, labor, and operation and maintenance costs (Table 5). The construction cost was approximately 26.76 USD/m², and the maintenance and management cost was approximately 4.07 USD/(m²•yr). This provides a reference for the treatment of other black odorous rivers.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.