Polyhydroxyalkanoates (PHAs) Production: A Feasible Economic Option for the Treatment of Sewage Sludge in Municipal Wastewater Treatment Plants?

Dafne Crutchik,Anuska Mosquera-Corral,Angeles Val Del Rio,José Luis Campos,Marisol Belmonte,Luis Caminos,Oscar Franchi,Alba Pedrouso,David Jeison

doi:10.3390/w12041118

Abstract

Sludge is a by-product of municipal wastewater treatment plants (WWTPs) and its management contributes significantly to the operating costs. Large WWTPs usually have anaerobic sludge digesters to valorize sludge as methane and to reduce its mass. However, the low methane market price opens the possibility for generating other high value-added products from the organic matter in sludge, such as polyhydroxyalkanoates (PHAs). In this work, the economic feasibility of retrofitting two types of WWTPs to convert them into biofactories of crude PHAs was studied. Two cases were analyzed: (a) a large WWTP with anaerobic sludge digestion; and (b) a small WWTP where sludge is only dewatered. In a two-stage PHA-production system (biomass enrichment plus PHAs accumulation), the minimum PHAs cost would be 1.26 and 2.26 US$/kg PHA-crude for the large and small WWTPs, respectively. In a single-stage process, where a fraction of the secondary sludge (25%) is directly used to accumulate PHAs, the production costs would decrease by around 15.9% (small WWTPs) and 19.0% (large WWTPs), since capital costs associated with bioreactors decrease. Sensitivity analysis showed that the PHA/COD (Chemical Oxygen Demand) yield is the most crucial parameter affecting the production costs. The energy, methane, and sludge management prices also have an essential effect on the production costs, and their effect depends on the WWTP’s size.

Highlights

Wastewater treatments plants (WWTPs) produce large quantities of sewage sludge, which requires adequate and environmentally safe management and disposal
Plant mass and energy balances were calculated for proposed scenarios for both studied WWTP
Results from the mass and energy balances related to net methane production, sludge generation, and energy consumption obtained for the large WWTP fit quite well to the actual data from the plant (Table 2)

Summary

Introduction

Wastewater treatments plants (WWTPs) produce large quantities of sewage sludge, which requires adequate and environmentally safe management and disposal. During the last few decades, sewage sludge management has moved from an approach involving only treatment and disposal, to its conversion into value-added products, such as bioenergy or biobased materials. The latter alternative has the potential to reduce the sludge quantity that needs to be. Water 2020, 12, 1118 disposed, and can decrease the overall operating costs [2] In this context, anaerobic digestion is a very useful technology for sludge management, since it can convert the biodegradable organic carbon into methane containing biogas [2,3]. PHAs are biodegradable polyesters synthesized by numerous bacteria that accumulate as intracellular carbon and energy reservoirs, under nutrient-limited growth conditions [5]

Methods

Results

Conclusion