The researchers studied how treating landfill leachate before mixing it with municipal wastewater affects the treatment process. They did a short-term study to mimic sudden leachate influx and a long-term study to see the effects over time. Pre-treating the leachate helped lower ammonia levels, improving nitrification. At low mixing ratios, pre-treatment didn't make a big difference, but at higher ratios, it helped with nitrification. Even with high ammonia levels in the leachate, the nitrification process wasn't negatively affected. However, the effluent quality decreased due to non-biodegradable COD in the leachate. Overall, a 2.5% mixing ratio improved nutrient removal without compromising COD removal.