**5. Algal blooms control in Tai-Hu Lake, China (too dirty)**

Taihu Lake, the third largest freshwater lake in China, is located in the highly developed and densely populated Yangtze River Delta. The lake is shallow with an average depth of 1.89 m, but a large surface area of 2,238 km2, and a volume of 4.66×109 m3. The annual water input averages 7.66×109 m3 and the residence time of its waters is about 300 days. There are over 219 inflow rivers or tributaries but only three main outflow rivers. The average of rainy days is 132 days/year and the average annual rainfall is about 1,145 mm. Rainfall varies seasonally with wet seasons between June and September, i.e., during the typhoon season or flood period, and the dry seasons from October to next May and constitutes the long dry period.

Taihu Lake plays multifunctional roles including floodwater storage, irrigation and navigation. It also serves as a major water resource for drinking, aquaculture and industrial needs, as well as being a source of entertainment and tourist interest. Its drainage basin extends over 37,000 km2 and is bounded by the Yangtze River to the north, the East China Sea to the east and mountainous areas to the west. While the basin accounts for 0.4% of the total area of China and 2.9% of the nation's population, it provides more than 14% of China's Gross Domestic Production (GDP). The GDP per capita is 3.5 times as much as the contry's average and its urbanization level ranks the first in China. This basin is vital for eastern China, where the lake water supports more than 60 million people (about 600–900 person/km2 on average), including the water supply to cities such as Wuxi, Suzhou, and Shanghai, one of the largest cities in the world.

With the tremendous economic growth and increased population in its basin, Taihu Lake has begun to suffer from various environmental stresses, including deterioration of its water quality with increasing nutrient and other chemical inputs. The lake is becoming increasingly eutrophicated and has experienced annual lake-wide cyanobacterial blooms in recent decades; this has affected the drinking water supply of surrounding cities. Taihu Lake now receives annually approximately 30,635,000 kg total nitrogen (TN) and 1,751,000 kg total phosphorus (TP) from a combination of municipal and industrial wastewaters and agricultural soil runoff; chemical oxygen demand on chromium (CODCr) is 131,223,000 kg (Qin et al, 2007).

Consequently, algae blooms have appeared and continued. The lake is often covered by algae blooms in summer, autumn and even spring. In 2007, a severe algae bloom caused a

Novel SPP Water Management Strategy and Its Applications 253

period. Instead they will always be closed even in the flood period if the river water is not clean enough. Thus, the first flush of each storm event will by-pass the lake in order to prevent the non-point source pollution. In the wet season, there is an average of 46.9 rainy days. The sluice gates will be opened, and only on these days will the floodwater be

> 46.9 / 365 1 /2 4 *<sup>o</sup> in <sup>o</sup> o <sup>W</sup> C C*

It can be seen that with the aid of sluice gates and BPC, the pollutant concentration entering to the lake can be significantly reduced. In other words, only 25% of contaminants yielded by its catchment in a year will be released into the lake to mix with the clean water while 75% of wastewater yielded in a year will by-pass the lake and be discharged to the downstream via the three outlets. While we have only discussed inflowing water concentrations, in principle, the concept can be extended to all other parameters, such as sediment inputs, BOD, TP, TN etc. Our estimation shows that if the SPP strategy is used, in about 3.5 years, the quality of lake water can be restored, the damaged eco-system can be

*Water Quality in BPC*: Taihu Lake has a residence time of 300 days and the slow water movements together with high concentrations of nutrients contribute to the problem of algal bloom in lakes. However, if the residence time of water is short, say 0.1 to 1 days, the high velocity of the water in the By-Pass Canal will prevent organic aggregation and transport phytoplankton into low light environments; turbulence will also keep phytoplankton and aggregates dispersed. Thus, there should be no problem of algal blooms in the canal. Higher water velocities do and can improve water quality in Taihu Lake, and this has been found in the lake: East Taihu Bay is a long (27.5 km) and narrow (greatest width is 9.0 km) bay located in east of Dongshan Peninsula; it connects with the West Taihu Lake at a narrow interface. East Taihu has an area of 132 km2 (5.9% of the total Lake Taihu surface area), with an average depth of only about 1.2 meters. The cross section of East Taihu Bay is much smaller relative to West Taihu Lake, but it is the main channel draining the lake. About 70% ~ 80% of the total outgoing discharge flows from this bay; therefore the flow velocity in this bay is higher than the velocity in the West Taihu Lake as this bay is much shallower and narrower. Similarly, water quality in the East Taihu Bay is better than the quality in the West Taihu Lake even the wastewater discharge received by the former bay is 4 to 5 times of the wastewater discharge received by the latter (Yang, 2004). This observation supports the inference that an increase of flow velocity can

From the above discussions, it is reasonable to conclude that the proposed scheme shown in Fig. 3 could significantly improve the water quality of Taihu Lake. Improvements are based on clean water being stored and protected by the inner levee while polluted water is retained (and concentrated) in the surrounding canal with algal blooms prevented by high flow speeds. Moreover, it is possible to further improve water quality in the canal by ecological remediation techniques and/or by flushing the wastewater in the canal using the clean water from the lake. High velocity water has strong ecological self-purification

remediated, and the algal blooms will disappear as nutrient levels decline.

improve the water quality.

capability.

*<sup>V</sup>* <sup>=</sup> <sup>≈</sup> (6)

discharged to the lake. The concentration Cin on these days is

drinking water contamination crisis for 4.43 million people in Wuxi city. An article in Science (Yang et al. 2008) reported that the concentration of dimethyl trisulfide in a water sample collected on 4 June 2007 from the drinking-water intake was 11,399 mg/liter—high enough to yield strong septic and marshy odors.

More than 50% of rainfall in the catchment appears from June to September, so this period can be defined as the wet season, and the remaining 8 months is the dry season. Unlike rainwater, industrial and municipal wastewater releases have little seasonal variation. The hydrograph of rainwater and wastewater is simplified as shown in Fig. 2. Integrating the runoff Qr with respect to time from January to December, one has:

$$
\int Q\_r dt = V\_o = 7.66 \times 10^9 \text{(m}^3\text{)}\tag{1}
$$

where *V*o = annual water yield in the basin.

From June to September, the water volume can be estimated as half of the total water yielded from the catchment as its rainfall is half of the annual rainfall, i.e.,

$$\int\_{lune}^{Sept} Q\_r dt \approx \frac{V\_o}{2} = 3.83 \times 10^9 \text{(m}^3\text{)}\tag{2}$$

Similarly, the wastewater yielded in the basin can be determined by

$$\mathcal{Q}\_w \times \text{365}(d) \times \text{86400}(s \ne d) = \mathcal{V}\_o \tag{3}$$

where *Wo* = yearly total volume of wastewater yielded in the basin and discharged to the waterways.

Currently all wastewater flows into the lake and its average concentration *Co* is

$$\mathcal{C}\_o = \frac{\mathcal{W}\_o}{V\_o} \tag{4}$$

In the wet season from June to September, the concentration *C*1 is

$$\mathbf{C}\_{1} = \frac{4\mathcal{W}\_{o} \;/\; 12}{V\_{o} \;/\; 2} = \frac{2}{3}\mathbf{C}\_{o} \tag{5}$$

where 4*Wo*/12 is the amount of wastewater yielded in 4 months (wet season), whilst Vo/2 is the amount of clean water yielded in the same period. Eq. 5 shows that in wet seasons, the water in inflow-rivers is relatively clean when compared to the water in dry seasons.

*Water Quality in the Lake*: Yang and Liu (2010) estimated the amount of wastewater entering the lake if the scheme shown in Fig. 3 is used. They assumed that river water with good quality is 50% of the total water resources, and it will be allowed to enter the lake via the sluice gates with the amount of 3.83×109m3, and the lake's storage capacity is the sum of the dead volume and the effective volume, i.e., 4.6×109 m3.

It should be stressed that while the rainwater from June to September flows into the lake via sluice gates, this does not mean that the sluice gates will remain open for the 4-month

drinking water contamination crisis for 4.43 million people in Wuxi city. An article in Science (Yang et al. 2008) reported that the concentration of dimethyl trisulfide in a water sample collected on 4 June 2007 from the drinking-water intake was 11,399 mg/liter—high

More than 50% of rainfall in the catchment appears from June to September, so this period can be defined as the wet season, and the remaining 8 months is the dry season. Unlike rainwater, industrial and municipal wastewater releases have little seasonal variation. The hydrograph of rainwater and wastewater is simplified as shown in Fig. 2. Integrating the

From June to September, the water volume can be estimated as half of the total water

where *Wo* = yearly total volume of wastewater yielded in the basin and discharged to the

*<sup>o</sup> <sup>o</sup> o*

4 /12 2 /2 3 *<sup>o</sup> <sup>o</sup>*

where 4*Wo*/12 is the amount of wastewater yielded in 4 months (wet season), whilst Vo/2 is the amount of clean water yielded in the same period. Eq. 5 shows that in wet seasons, the

*Water Quality in the Lake*: Yang and Liu (2010) estimated the amount of wastewater entering the lake if the scheme shown in Fig. 3 is used. They assumed that river water with good quality is 50% of the total water resources, and it will be allowed to enter the lake via the sluice gates with the amount of 3.83×109m3, and the lake's storage capacity is the sum of the

It should be stressed that while the rainwater from June to September flows into the lake via sluice gates, this does not mean that the sluice gates will remain open for the 4-month

*o <sup>W</sup> C C*

water in inflow-rivers is relatively clean when compared to the water in dry seasons.

*<sup>W</sup> <sup>C</sup>*

9 3 3.83 10 ( ) <sup>2</sup>

9 3 7.66 10 ( ) ∫*Q dt V r o* = = × *<sup>m</sup>* (1)

∫ *Q dt* ≈= × *<sup>m</sup>* (2)

365( ) 86400( / ) *Q d sd W w o* × × = (3)

*<sup>V</sup>* <sup>=</sup> (4)

*<sup>V</sup>* = = (5)

enough to yield strong septic and marshy odors.

where *V*o = annual water yield in the basin.

waterways.

runoff Qr with respect to time from January to December, one has:

yielded from the catchment as its rainfall is half of the annual rainfall, i.e.,

Similarly, the wastewater yielded in the basin can be determined by

In the wet season from June to September, the concentration *C*1 is

dead volume and the effective volume, i.e., 4.6×109 m3.

*Sept <sup>o</sup> <sup>r</sup> June*

Currently all wastewater flows into the lake and its average concentration *Co* is

1

*V*

period. Instead they will always be closed even in the flood period if the river water is not clean enough. Thus, the first flush of each storm event will by-pass the lake in order to prevent the non-point source pollution. In the wet season, there is an average of 46.9 rainy days. The sluice gates will be opened, and only on these days will the floodwater be discharged to the lake. The concentration Cin on these days is

$$\mathbf{C}\_{in} = \frac{46.9 W\_o \, / \, 365}{V\_o \, / \, 2} \approx \frac{1}{4} \mathbf{C}\_o \tag{6}$$

It can be seen that with the aid of sluice gates and BPC, the pollutant concentration entering to the lake can be significantly reduced. In other words, only 25% of contaminants yielded by its catchment in a year will be released into the lake to mix with the clean water while 75% of wastewater yielded in a year will by-pass the lake and be discharged to the downstream via the three outlets. While we have only discussed inflowing water concentrations, in principle, the concept can be extended to all other parameters, such as sediment inputs, BOD, TP, TN etc. Our estimation shows that if the SPP strategy is used, in about 3.5 years, the quality of lake water can be restored, the damaged eco-system can be remediated, and the algal blooms will disappear as nutrient levels decline.

*Water Quality in BPC*: Taihu Lake has a residence time of 300 days and the slow water movements together with high concentrations of nutrients contribute to the problem of algal bloom in lakes. However, if the residence time of water is short, say 0.1 to 1 days, the high velocity of the water in the By-Pass Canal will prevent organic aggregation and transport phytoplankton into low light environments; turbulence will also keep phytoplankton and aggregates dispersed. Thus, there should be no problem of algal blooms in the canal. Higher water velocities do and can improve water quality in Taihu Lake, and this has been found in the lake: East Taihu Bay is a long (27.5 km) and narrow (greatest width is 9.0 km) bay located in east of Dongshan Peninsula; it connects with the West Taihu Lake at a narrow interface. East Taihu has an area of 132 km2 (5.9% of the total Lake Taihu surface area), with an average depth of only about 1.2 meters. The cross section of East Taihu Bay is much smaller relative to West Taihu Lake, but it is the main channel draining the lake. About 70% ~ 80% of the total outgoing discharge flows from this bay; therefore the flow velocity in this bay is higher than the velocity in the West Taihu Lake as this bay is much shallower and narrower. Similarly, water quality in the East Taihu Bay is better than the quality in the West Taihu Lake even the wastewater discharge received by the former bay is 4 to 5 times of the wastewater discharge received by the latter (Yang, 2004). This observation supports the inference that an increase of flow velocity can improve the water quality.

From the above discussions, it is reasonable to conclude that the proposed scheme shown in Fig. 3 could significantly improve the water quality of Taihu Lake. Improvements are based on clean water being stored and protected by the inner levee while polluted water is retained (and concentrated) in the surrounding canal with algal blooms prevented by high flow speeds. Moreover, it is possible to further improve water quality in the canal by ecological remediation techniques and/or by flushing the wastewater in the canal using the clean water from the lake. High velocity water has strong ecological self-purification capability.

Novel SPP Water Management Strategy and Its Applications 255

Fig. 7. The Lake Dianchi basin in China

Fig. 8. The Biwa Lake, Japan (after Mori et al. 1984)
