4.1. Treatment Performance of the Wastewater Irrigated Short Rotation Coppice System in a Cold Climate
The results demonstrate that a short rotation coppice system with Willow and Poplar can function as a wastewater treatment and wood production technology in cold climate regions under two different seasonal conditions. Whereas a number of studies have been carried out thus far under conditions similar to bed A with external winter storage, bed B with internal winter storage has distinct differences with respect to any other waste water irrigated short rotation coppice system reported in the literature.
Despite having long and cold winters, Mongolia also experiences short and hot summers, which has proven to lead to high ET in such systems [1
]. For bed A, this high ET leads to minimal drainage water, which positively influences the treatment performance of the wastewater. A similar bed, fed throughout the year with a much smaller amount of pre-treated wastewater (5 mm·day−1
), showed an ET which was very similar to bed B operated throughout the year [1
]. The slightly higher ET found in bed B compared to bed A might be explained by the fact that there was no ice accumulation followed by a peak drainage from this particular bed. The ET recorded during this study is consistent with that found by [14
] in Scandinavia and [15
] in Italy, proving that this wastewater treatment system can function with high efficiency even in colder temperate countries, such as Mongolia.
In terms of organic load, bed A demonstrated both high mass removal rate and removed percentage while bed B showed much higher mass removal rate but low removed percentage. In the US-EPA [3
] it is stated that the main removal process for COD and BOD5
in wastewater land treatment systems occurs on the soil surface or close to the soil surface layer and that the application rate of 500 kg BOD5
is not a limiting factor for a slow rate land treatment system. Paranychianakis et al. [16
] reported an efficient removal rate of more than 90% at a BOD application rate of 330 kg·ha−1
. In the present study, the maximum application rates of 18 kg COD·ha−1
and 11 kg BOD5
were much lower than the reported values in published data, suggesting that the treatment beds were not overloaded. On the contrary, the low removal percentage found in bed B, which was operated throughout the year, reflects the peak drainage and the contained mass of COD and BOD5
In the present study, the mean annual application rate with regard to TN and TP for bed A with external winter storage was as high as 1050 and 55 kg·ha−1
, respectively. Thereby, it is in the same range to that described for land treatment systems with a mean annual application rate of TN and TP of 1170 and 104 kg·ha−1
, respectively [17
]. Bed B with internal winter storage, operated throughout the year, was fed with a much higher load of nutrients; 2190 and 187 kg·ha−1
TN and TP, respectively. Tzanakakis et al. [18
] and Holm and Heinsoo [19
] reported higher removal efficiencies for TN and TP (95% and 84%, respectively) at this range of nutrient load, which might be explained by the fact that these studies were conducted in Greece and Estonia, which have a warmer year-long climate compared to Mongolia. Bed B, showing TN removal percentage of only 35–37%, might suggest that this particular bed may have been overloaded with nutrients. Another bed operated under same condition (of internal winter storage) with a lower loading rate (of 5 mm·day−1
) exhibited TN and TP removal percentage of 80% and 85%, respectively [1
]. In general, both beds showed high mass removal rates, highlighting the removal capacity of such a system in a cold climate.
In general, land treatment systems are distinguished by a high treatment performance for E. coli [3
]. The low treatment performance shown in the present study in the case of bed B might be explained by the unique character of the wastewater treatment system, which accumulates the wastewater during wintertime, followed by the ice melt effect. Tzanakakis et al. [11
] reported in his study a reduction of up to 6 logs for E. coli, whereas this investigation (similar to other studies) was conducted under much warmer climatic conditions and at lower loading rates.
One of the benefits of the wastewater treatment technology described here is the simultaneous production of biomass, which is in high demand in Mongolia, considering the cold winters. Conventional short rotation forestry systems have a biomass yield of 6–8 t DM·ha−1
]. When irrigated with pre-treated wastewater, the biomass yield often increased reaching up to 15 t DM·ha−1
]. In the present study, without considering the edge effect of scaling up, the biomass yield was as high as 28 and 34 t DM·ha−1
for bed A with external winter storage and bed B with internal winter storage, respectively. This might have been the result of high water availability, as the same bed at a lower loading rate and during the same period of operation exhibited a biomass yield of 13 t DM·ha−1
]. However, as in the present study, a high biomass yield was reported by [23
] for a short rotation intensive forestry coppice systems in the U.S.
4.2. Design Recommendations for Mongolia
Based on the experimental results of the present study and the earlier studies presented in Khurelbaatar et al. [1
], the following three major options for up-scaling the wastewater irrigated short rotation coppice systems are recommended to serve as a potential wastewater treatment and biomass production solution for a Mongolian village. Considerations for scaling up must be taken into account, since the recommendation is solely based on the results of a pilot project and the following numbers are extrapolated without considering the edge effects. According to Dolgorsuren et al. [24
], there are 64 communities in Mongolia where the treated wastewater is applied to infiltration basins. These treatment plants are, however, either under critical conditions or not in operation. The short rotation coppice systems with wastewater irrigation would be an ideal and “easy-to-build” improvement of the existing situation in terms of both environmental and economical requirements.
As an example, a town of 1500 PE (close to Darkhan city) was chosen to demonstrate the three potential designs of a short rotation coppice system for wastewater treatment and biomass production. MoMo-I [25
] reported an average drinking water consumption of 265 L·PE−1
in Darkhan city. For calculating the wastewater generation of the village, the amount of water consumption per person is multiplied by a factor of 0.825 [26
], which resulted in 218.6 L·PE−1
. Assuming that the WWTP should serve 1500 inhabitants, around 120,000 m3
of wastewater should be applied to the system annually. The annual load was calculated based on the estimated hydraulic load and the concentration of primary treated wastewater (see Chapter 2.2).
The three main options presented in Figure 7
are based on the mass removal rates of bed A with external winter storage and bed B with internal winter storage. Another option C with internal winter storage and lower loading rate was included, which was based on a prior study [1
], For option B, no external winter storage is required, assuming that the treatment area will serve as an internal winter storage, whereas the design option A includes external winter storage. The dimensions of the winter storage beds were calculated, estimating that 2/3 of the annual wastewater amount had to be stored. This resulted in a storage area of 3 ha with a depth of 2.7 m.
For each option, the area of the treatment fields was calculated based on the respective hydraulic loading rate (of 15, 15 and 5 mm·day−1
for options A, B and C, respectively. Options A and C have similar treatment areas, while option B requires a much smaller area. In spite of the different treatment areas, design options C and B offer similar yields of biomass, whereas option A offers an around two-fold biomass yield. The amount of pollutants associated with drainage water is lowest for option A and highest for option B. Figure 7
presents the mass loadings for primary treated wastewater, the required surface area for the treatment beds, the potential mass of pollutants in drainage water, and the potential biomass yields for each design option.
The irrigation system is not intensively studied in this paper. Most of the irrigation systems used in land application and short rotation forestry systems in other countries are not applicable to the Mongolian situation, which refers in particular to option B and C. The most common and robust irrigation would be a furrow (trench) irrigation system. In the case of option A, pipe and/or drip irrigation can be used. Attention must however be paid to the necessity of emptying the irrigation system before winter.
In addition, it has been observed that the irrigation period for option A can be extended up to 6 months starting in mid-April and ending in mid-October, resulting in smaller winter storage, making this option even more attractive in terms of investment cost.
Option A is recommended for town and municipalities seeking the highest amount of biomass yield accompanied by the lowest risk for groundwater contamination and high investment for the external winter storage. Option B is recommended for municipalities, which have limited space. In spite of the lower reduction rates for pollutants compared to the other two options, it offers a high yield of biomass for a small area. In order to avoid groundwater contamination, when applying this option, special attention should be paid to the nutrients. The recommended option C is suitable for municipalities with enough available space. By far, it is also the option to reduce the pollutants to a high degree and recycle the wastewater for biomass production.