Evaluating the economic viability and environmental impact of emerging technologies is crucial for the transition to a bio-based economy. This study proposes a methodology to assess the environmental and economic performance of bio-based lactic acid (LA) production by scaling up from pilot to industrial levels using fiber sludge, a residue from the pulp and paper industry, as a feedstock. Process design, Techno-Economic Analysis (TEA) and Life Cycle Assessment (LCA) were conducted at pilot scale to identify key environmental and economic hotspots. External costs were estimated following the environmental Life Cycle Costing (eLCC) approach using the Environmental Prices (EP) method. At the pilot scale, the LCA indicated a Global Warming Potential (GWP) of 3.87 kg CO₂-eq, which aligns with the values reported in previous studies. Scaling up to different plant capacities revealed the potential economies of scale. At a production rate of 50 kt per year, the Minimum Selling Price (MSP) was estimated at 1.71€/kg, which is comparable to that of other bio-based LA production routes. Assuming proportional environmental impacts from pilot to industrial scale, external costs were integrated into the MSP, resulting in adjusted values of 2.04€/kg (lower value), 2.21€/kg (central value), and 2.46 €/kg (upper value). Sensitivity and uncertainty analyses using Monte Carlo simulations indicated an 87.5 % probability of achieving a positive Net Present Value (NPV). This study highlights the need for standardised methodologies to evaluate the environmental and economic impacts of emerging bio-based technologies, particularly when accounting for external costs