The growing recognition of the potential of recovery of products from, and recycling of, wastewater is matched by a growing interest in decentralized treatment systems. Sanitation-focused policies and interventions, undertaken by many developing countries, are dominated by septic tanks for septage management. As reserves of phosphate rock continue being depleted, recovery of phosphorus (P) from the waste stream is one of the practical strategies to deal with that depletion. Partial anaerobic digestion of septic tank liquor converts organic P to soluble orthophosphate form, allowing it to be recovered. Factors that influence such recovery, namely pH, the molar ratio of magnesium (Mg) to P, velocity gradient, and mixing duration were tested and optimized for recovering P as struvite. Interactions between the above factors and their possible influence on the efficiency of P recovery were also examined. Under optimal conditions, more than 90% of P could be recovered. The maximum size of struvite crystals was 201 μm. Among the factors that influenced P recovery, pH and the Mg/P ratio affected recovery efficiency whereas the velocity gradient and mixing duration affected the size of struvite crystals. Tapering the velocity gradient not only increased the crystal size significantly, from 280 μm to 371.9 μm, but also reduced energy requirement by 62%. The study showed that new reactors designs can be based on the concept of tapered velocity gradient to improve the properties of struvite crystals.