EWLS: A New Local Search for Minimum Vertex Cover

A number of algorithms have been proposed for the Minimum Vertex Cover problem. However, they are far from satisfactory, especially on hard instances. In this paper, we introduce Edge Weighting Local Search (EWLS), a new local search algorithm for the Minimum Vertex Cover problem. EWLS is based on the idea of extending a partial vertex cover into a vertex cover. A key point of EWLS is to find a vertex set that provides a tight upper bound on the size of the minimum vertex cover. To this purpose, EWLS employs an iterated local search procedure, using an edge weighting scheme which updates edge weights when stuck in local optima. Moreover, some sophisticated search strategies have been taken to improve the quality of local optima. Experimental results on the broadly used DIMACS benchmark show that EWLS is competitive with the current best heuristic algorithms, and outperforms them on hard instances. Furthermore, on a suite of difficult benchmarks, EWLS delivers the best results and sets a new record on the largest instance

Optimal production of polyhydroxyalkanoates (PHA) in activated sludge fed by volatile fatty acids (VFAs) generated from alkaline excess sludge fermentation

To reduce the production cost of polyhydroxyalkanoates (PHA) and disposal amount of excess sludge simultaneously, the feasibility of using fermentative volatile fatty acids (VFAs) as carbon sources to synthesize PHA by activated sludge was examined. At pH 11.0, 60 °C and fermentative time of 7 d, the VFAs yield was 258.65 mgTOC/gVSS. To restrain cell growth during PHA production, the released phosphorus and residual ammonium in the fermentative VFAs was recovered by the formation of struvite precipitation. Acetic acid was the predominant composition of the fermentative VFAs. PHA accumulation in excess sludge occurred feeding by fermentative VFAs with aerobic dynamic feeding process. The maximum PHA content accounted for 56.5% of the dry cell. It can be concluded from this study that the VFAs generated from excess sludge fermentation were a suitable carbon source for PHA production by activated sludge