Phycoremediation by High-Rate Algal Ponds (HRAPs) - Biotechnological Applications of Microalgae - page 192

removal efficiencies of up to 98%, 100%, and 72.6%, respectively, for the treatment
of municipal wastewater. Nutrient removal efficiencies depend on the cultural con-
ditions as mentioned previously and the nutrient loading rate. Boelee et al . (2011)
showed a linear increase in nitrate and phosphate uptake with increasing loading rate
up to 1.0 g m −2 d −1 and 0.13 g m −2 d −1 , respectively, from municipal wastewater. Wang
et al . (2011) showed an ammonia removal rate of 90%, irrespective of the initial con-
centration used. Furthermore, total nitrogen and phosphorus was found to be greatly
reduced from piggery wastewater. Nutrient removal efficiencies ranging from 91% to
96% ammonia and 72% to 87% phosphate, depending on the season and depth of the
culture, were observed by Olguin (2003).
12.6 WASTEWATER AS FEEDSTOCK FOR BIOMASS PRODUCTION
Microalgal wastewater treatment using microalgae with the production of biomass
as a by-product is not a new concept. However, it occurs only on a minor scale
in waste stabilization ponds and HRAPs. Wastewater treatment using HRAPs has
the potential to produce large amounts of biomass that can be used for a variety of
applications, including the production of renewable fuels, fertilizer, animal feed, etc.
(Rawat et al . , 2011). Recent studies have suggested that the use of wastewater as a
substrate for biofuel production may make the process economically viable (Brennan
and Owende, 2010; Boelee et al . , 2011; Cho et al . , 2011). Focusing the growth of
microalgae on biomass productivity rather than lipid productivity may be beneficial
as larger amounts of biomass improve the viability of conversion to alternate fuels
(Pittman et al . , 2011). Microalgal biomass to biofuels conversion may be carried out
by several methods depending on the biomass characteristics (e.g., lipid or carbohy-
drate content) (García et al., 2006; Rawat et al . , 2011). The yields of biomass from
HRAPs depend on the type of effluent being treated with specific regard to nutrient
content. Table 12.2 summarizes growth and lipid productivity of microalgal species
on a variety of wastewater types. Piggery waste effluent treatment by HRAPs has
potential productivities of up to 50 t ha −1 yr −1 (Rawat et al . , 2011).
Maximum algal productivities in HRAPs can be achieved by countering rate-
limiting and inhibitory conditions. Carbon is often a rate-limiting substrate and may
be alleviated by the addition of CO 2 . This addition serves a dual role in the provi-
sion of carbon and a method of pH control. The addition of CO 2 has been shown to
double algal productivity at the laboratory scale and increase productivity by 30% in
a pilot-scale HRAP (Park et al . , 2011a). Biomass grown at the Lawrence wastewater
treatment plant showed algal productivities ranging from 5 to 16 g m −2 d −1 and average
lipid contents of 10% without the addition of CO 2 . With the addition of CO 2 , produc-
tivities were expected to be 25 g m −2 d −1 (Sturm and Lamer, 2011). However, it must
be considered that addition of excess CO 2 leads to a decrease in pH. A pH maintained
at a maximum of 8 inhibits physico-chemical processes of nutrient removal such as
volatilization of ammonia and phosphate precipitation (Craggs, 2005). But this is not
necessarily a negative point, as the increase in assimilation by biomass production
offsets the losses on physico-chemical removal. Furthermore, it enables the recycling
of nutrients that would have been otherwise lost. Ammonia volatilization accounts for
approximately 24% nitrogen loss in HRAPs without pH control (Park et al . , 2011a).