Physical and Chemical Processes for Removing Suspended Solids and Phosphorus from Liquid Swine Manure

Abstract

A physical/chemical treatment train, that included 24-hour preliminary settling followed by coagulation/flocculation and sedimentation, was tested at a laboratory bench scale to treat liquid swine manure for the removal of total suspended solids (TSS) and total phosphorus (TP). Preliminary (i. e., natural) settling time had an effect on TSS removal within only the first 24 hours. TSS removal efficiency reached 75% (TSS concentration was reduced from 5,800 to 1,450 mg l-1) after 24 hours of preliminary settling. Also, as a result of the 24-hour preliminary settling, TP concentration was reduced from 533 to 318 mg l -1, thus leading to a TP removal efficiency of 40%. When compared to ferric chloride, alum was more effective in reducing both TSS and TP. At a 95% confidence interval, alum dose, coagulation Gt (coagulation velocity gradient * rapidmixing time), and flocculation Gt (flocculation velocity gradient * slow-mixing time) were not significant for TSS removal while alum dose was the only significant factor for TP removal. For the 24-hour settled liquid manure that had a TP concentration in the range of 362 to 401 mg l-1 and as alum dose increased up to 1,600 mg l-1, TP removal efficiency increased up to 70%. Then, the rate of increase in TP removal efficiency per increase in alum dose started to decrease and TP removal efficiency reached about 93% at an alum dose of 3,000 mg l-1. Sequential alum dosing improved the TSS removal efficiency while it had no effect on TP removal efficiency. The mass ratio of removed TSS/applied alum increased from about 0.38, during a one-step dosing of alum at a concentration of 1,600 mg l-1, to about 0.58 during a two-step dosing of alum at a concentration of 1,600 mg l-1 (i. e., 800 mg l-1 followed by another 800 mg l-1).